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Ceramic processing of incinerator bottom ash
Waste Management 23 (2003) 907–916 www.elsevier.com/locate/wasman
Ceramic processing of incinerator bottom ash C.R. Cheesemana,*, S. Monteiro da Rochaa, C. Sollarsa, S. Bethanisa, A.R. Boccaccinib a
Department of Civil and Environmental Engineering, Centre for Environmental Control and Waste Management, Imperial College of Science, Technology and Medicine, London SW7 2BU, UK b Department of Materials, Imperial College of Science, Technology and Medicine, London SW7 2BP, UK Accepted 25 February 2003
Abstract The < 8 mm fraction of aged incinerator bottom ash from a commercial incinerator (energy from waste) plant has been collected at regular intervals, characterised and processed to form ceramic materials. Ashes were sieved, wet ball milled, dried, compacted and sintered at temperatures between 1080 and 1115 C. Variations in the chemical composition and mineralogy of the milled ash, and the mineralogy, physical properties and leaching of sintered products have been assessed. Milling produces a raw material with consistent chemical and mineralogical composition with quartz (SiO2), calcite (CaCO3), gehlenite (Ca2Al(AlSi)O7) and hematite (Fe2O3) being the major crystalline phases present. Different batches also milled to give consistent particle size distributions. Sintering milled incinerator bottom ash at 1110 C produced ceramics with densities between 2.43 and 2.64 g/cm3 and major crystalline phases of wollastonite (CaSiO3) and diopside (CaMgSi2O6). The sintered ceramics had reduced acid neutralisation capacity compared to the as-received ash and exhibited reduced leaching of Ca, Mg, Na and K under all pH conditions. The leaching of heavy metals was also significantly reduced due to encapsulation and incorporation into glassy and crystalline phases, with Cu and Al showing greatly reduced leaching under alkali conditions. # 2003 Elsevier Ltd. All rights reserved.
1. Introduction Many countries increasingly consider incineration as an essential part of integrated management strategies for municipal solid waste (MSW). This is often driven by limited availability of landfill capacity and the advantages offered by incineration in modern ‘energy from waste’ (EfW) plants. Incineration reduces the volume of MSW by about 90%, but still produces signicant amounts of incinerator bottom ash (IBA) that need to be carefully managed. As MSW incineration and the resulting volumes of IBA increase, there is increasing need to develop new reuse applications for this material (Woolley et al., 2001; Chang et al., 1999). In England and Wales, approximately 28 million tonnes of MSW are currently produced each year, of which about 10% are currently incinerated in 14 operational EfW * Corresponding author. Tel.: +44-207-594-5971; fax: +44-207823-9401. E-mail address: [email protected] (C.R. Cheeseman). 0956-053X/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0956-053X(03)00039-4
plants. This produces in excess of 800 kilotonne of IBA, most of which is landfilled, although the potential for re-use of size sorted IBA in civil engineering applications is being extensively investigated. In some European countries such as The Netherlands, Denmark, Germany and France, processed bottom ash is routinely used in various construction projects (Wainwright, 1981; Wainwright and Boni, 1983; Sawell et al., 1995; Pera et al., 1997; Williams, 1998). IBA is a highly heterogeneous mix of slag, ferrous and non-ferrous metal, ceramics, glass, other non-combustibles and residual organic matter. The composition is directly related to the composition of the waste being incinerated and the sources of various elements in MSW are diverse and influence the characteristics of the IBA produced. For example, heavy metals such as cadmium, lead, mercury and zinc are mainly found in printing inks, paints, pigments, plastics and household batteries, while other wastes such as foil, cans, glass, ferrous and non-ferrous-materials may contain aluminium, chromium, copper, iron, nickel and alkaline-earth metals (Chimenos et al., 1999).
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At incinerator temperatures of 900–1000 C some elements volatilise and are collected in the air pollution control (APC) residues. The efficiency and type of incinerator influences partitioning and poor combustion can result in relatively high concentrations of volatile trace metals in IBA. The organic matter in IBA is mainly composed of un-combusted cellulose (74%) and lignin (20%) derived from the large amounts of paper, wood and other plant material found in MSW (Pavasars, 2000). Sintering is a high temperature process that produces strength and other engineering properties in compacted fine powdered materials through particle bonding and densification. It is widely used in industry to produce a diversity of materials, including ceramics, metals and composites. Sintering has the potential to convert a range of inorganic waste ashes such as MSW IBA into environmentally benign new solid-monolithic ceramic products with properties that may allow reuse in appropriate applications (Boccaccini et al., 2000a). The inherent variability of IBA means that processing prior to sintering is essential if consistent ceramic materials are to be produced. Although the effects of sintering incinerator fly ash have been investigated, the properties of sintered IBA have not been widely reported (Wang et al., 1998; Wainwright and Cresswell, 2001; Bethanis et al., 2002). IBA has been used as a raw material combined with glass cullet for the production of glass–ceramics, and glass–ceramics have been formed from mixes containing IBA, glass and an industrial waste fly ash (Barbieri et al., 2000a, b). Glass matrix composites have also been produced from IBA combined with aluminium foundry waste (Ferraris et al., 2001). The glass–ceramic production process involves high temperature melting (vitrification) combined with subsequent heat treatment to induce devitrification. The process investigated in this work is different, produces different materials and involves direct sintering of processed IBA at temperatures significantly below those needed for vitrification. The main objectives of this work were to investigate the properties of materials made by applying ceramic processing to IBA and to assess the effects of IBA variability. Materials obtained at different stages of the process have been characterised and the leaching of as-received IBA and sintered IBA compared.
mass burn incineration technology with controlled high temperature combustion combined with advanced fluegas cleaning. The IBA produced is transported off-site, weathered for 6–8 weeks and the readily extractable ferrous and non-ferrous metals removed for recycling. The remaining material is screened into different sizes as the coarse fractions have potential uses as aggregate and inert fill. There are currently no viable re-use applications specifically for the < 8 mm fraction of IBA although this represents approximately 45 wt.% of the total ash. Representative samples of weathered IBA from which readily extractable metals had been removed were collected over a 10-week period during 2001. Each of the six batches collected was screened through an 8 mm sieve and the material passing used throughout subsequent experiments. Variability in the particle size distribution of the < 8 mm IBA samples was determined by sieve analysis using 150, 355 mm, 1.70 and 3.35 mm ASTM standard sieves. 2.1.2. Production of milled bottom ash Each batch of < 8 mm IBA was oven dried overnight at 105 C and wet ball milled for 8 h in a 3-l polypropylene mill, rotating at 50 rpm, using high-density alumina media. 500 g batches of ash were milled at a water-to-ash ratio of 2. Slurries obtained from milling were passed through 1.70 mm and 355-mm sieves to remove coarse particles and the coarse material retained by the 1.70-mm sieve was dried and weighed. The < 355 mm fraction was de-watered by pressure filtration in a stainless steel extraction vessel on Whatman GF/C filter paper. The filter cake obtained was oven dried overnight at 105 C, ground in a pestle and mortar and passed through a 150-mm sieve to produce a fine homogeneous grey powder suitable for sintering. 2.1.3. Production of sintered ceramic samples The dried milled powder was uni-axially compacted at 32 MPa in a stainless steel die to form cylindrical ‘green’ samples, approximately 20 mm in diameter and 12–15 mm in height, without the addition of an organic binder or other additives. These were sintered in an electric furnace at temperatures between 1080 and 1120 C using a ramp rate of 6 C min1 with a dwell time of 60 min at the maximum temperature. 2.2. Materials characterisation
2. Materials and methods 2.1. Materials processing 2.1.1. Sampling MSW IBA from a major EfW plant situated in southeast England has been used in this work. This plant processes approximately 420 000 tonnes of waste per year and generates 35 MWh of electricity. It uses well-established
2.2.1. Characterisation of milled IBA The particle size distribution between 0.4 and 900 mm in each batch of milled slurry was analysed by laser diffraction analysis (Beckman Coulter LS-100). 2.2.2. X-Ray powder diffraction The crystalline phases present in the dried milled ash batches were qualitatively identified by X-ray diffraction
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(XRD) using a Philips PW1830 diffractometer system fitted with a PW1820 goniometer, an automatic divergence slit and a graphite monochromator with CuKa radiation at an accelerating voltage of 40 kV. 2.2.3. Chemical characterisation The elemental composition of dried milled bottom ash was determined by digestion using lithium metaborate and tetraborate flux fusion (Ingamells, 1970). Digests were analysed by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The weight loss was determined by heating 4-g samples to 1000 C for 12 h. This includes the weight loss due to combustion of organic matter, dehydration and the degradation of carbonated species formed by atmospheric exposure. 2.2.4. Characterisation of sintered IBA samples The density of sintered samples was determined using Archimedes’ method and the linear shrinkage calculated by comparing sample diameters before and after firing. Water absorption of sintered IBA was determined from the weight increase of surface dry samples after immersion in water for 24 h (BS 812, 1975). Samples of sintered IBA were ground to < 150mm and analysed by XRD to identify the crystalline phases present. The methodology and equipment used was the same as for XRD analysis of milled bottom ash samples. Scanning electron microscopy (SEM, Jeol JSM-T200) was used to examine the microstructure of carbon coated polished surfaces and fracture surfaces of IBA samples sintered at 1100 C. Samples were also analysed for elements present in different phases by energy dispersive X-ray (EDX) spectroscopy. The Vickers micro-hardness of IBA samples sintered at 1100 C was measured using a Leitz Wetzlar 8423 microhardness tester with 25 g load. Prior to hardness testing the samples were embedded in epoxy resin and polished to a 0.5 mm diamond finish. Average hardness values were calculated from five indents on sintered samples from batches 1, 4 and 5. The Vickers micro-hardness test uses a square-based diamond pyramid with opposite sides meeting at the apex at an angle of 136 . The diamond is pressed into the surface of the material and the size of the impression measured using a calibrated microscope. The Vickers number (HV), given in units of kg mm2, is calculated from: HV ¼ 1:854 F=D 2 ;
where F is the applied load (kgf) and D2 the area of the indentation measured in mm2 (Ullner et al., 2001).
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2.3. Metal leaching from as-received and sintered IBA The acid neutralisation capacity (ANC) test, combined with leachate analysis, was completed on all batches of as-received IBA and IBA sintered at 1100 C (Stegemann and Coˆte´, 1991). This test allows the leaching under different pH conditions to be assessed. Samples for leaching were dried and ground to pass through a 150-mm sieve and 5-g samples mixed with 30 ml of aqueous acid solutions that varied in concentration between 0 and 100% 2.0 N nitric acid over 11 equal increments. The slurries obtained were mixed for 48 h in a rotary extractor and then centrifuged at 6000 rpm for 10 min to separate the leachate. The leachant/ sample contact time of 48 h allows the solids to equilibrate and centrifugation reduces interference in pH analysis. The leachate was extracted and filtered through a 0.45-mm membrane filter and acidified with 10% volume HNO3 prior to analysis by ICP–AES for a range of elements including Co, Cd, Cr, Cu, Fe, Mn, Ni, Pb, and Zn.
3. Results and discussion 3.1. As-received and milled IBA characterisation The sieve analysis data for the different batches of < 8 mm IBA is shown in Fig. 1. This indicates fairly consistent results at size fractions smaller than 355 mm, while at larger particle sizes significant variability occurs, with Batch 5 exhibiting lower content of coarse particles than the other batches. After milling for 8 h the slurry was passed through 1.70mm and 355-mm sieves and the residue collected on the sieves washed and dried. The amount greater than 1.7 mm
Fig. 1. Particle size distribution of the as-received batches of <8 mm incinerator bottom ash.
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Fig. 2. Percentage weight of coarse material retained on a 1.70-mm sieve after 8 h wet ball milling of <8 mm IBA.
is shown for each batch in Fig. 2. This typically represents about 30% of the original < 8 mm fraction and mainly consists of rounded glass and other difficult to mill materials, such as rock, bone, wood, some plastic and metal. It is a processing by-product that has potential to be used after further screening as normal weight aggregate. The particle size distribution of the different IBA batches after milling for 8 h is shown in Fig. 3. All batches show very similar distributions, with typically 50% of the particles with diameter finer than 8 mm and 95% less than 50 mm. Particle size distribution is an important factor in ceramics production as this has a key role in controlling sintering. It is expected that the 8-h milling time required to produce this type of particle size distribution could be significantly reduced using larger, commercial scale ball milling equipment.
XRD data for different milled IBA batches is shown in Fig. 4. This shows that the major crystalline phases qualitatively identified in the different milled batches are very similar, indicating that it may be possible to produce a consistent raw material by processing IBA. The major phases identified are quartz (SiO2) and calcite (CaCO3), with a minor presence of gehlenite (Ca2Al(AlSi)O7) and Table 1 Chemical composition of different batches of milled incinerator bottom ash Component Batch 1 Batch 2 Batch 3 Batch 4 Batch 5 Batch 6 Major elements (wt.%) SiO2 34.29 Al2O3 10.83 Fe2O3 7.38 MgO 2.16 CaO 20.80 Na2O 2.42 K2O 0.83 1.58 P2O5 TiO2 1.24 MnO 0.11
34.27 10.89 7.46 2.08 21.21 2.16 0.82 1.58 1.34 0.10
34.91 10.45 7.90 2.19 21.34 2.40 0.81 1.65 1.34 0.11
35.61 10.28 7.77 2.04 20.78 2.36 0.83 1.60 1.25 0.13
35.52 10.50 7.87 2.13 20.47 2.45 0.85 1.60 1.29 0.11
34.31 10.78 7.69 2.09 20.52 2.32 0.82 1.56 1.23 0.11
LOIa
15.76
14.62
14.88
14.43
15.28
15.01
Minor and trace elements content (mg/kg) Ba 1045 1030 1150 Be 0.80 1.2 1.4 Co 20 20 25 Cr 445 340 355 Cu 2000 2000 1900 Ni 160 100 110 Sc 2 4 4 Sr 410 350 350 V 45 35 35 Y 6 8 10 Zn 2800 4000 2500 Zr 160 175 180 Fig. 3. Particle size distribution of batches of <8 mm IBA after milling for 8 h.
a
LOI, loss on ignition.
1120 1.2 20 390 2150 145 4 350 35 8 3300 175
1060 1.4 20 355 1840 120 4 350 35 10 2700 180
1030 1.2 20 315 1950 110 4 360 30 8 3300 175
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hematite (Fe2O3) and these minerals have previously been identified as major components of incinerator bottom ash (Pfrang-Stotz et al., 2000; Zevenbergen et al., 1994). Chemical composition data for the different batches of milled IBA is shown in Table 1. All milled batches show very consistent composition in terms of major oxides, with approximately 70% of the weight consisting of silica, alumina, iron and calcium oxides. The concentrations of trace elements are also very similar, with the exception of Zn, which shows relatively high variability (2500–4000 mg/kg). Heavy
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metals of particular environmental concern present at high concentrations include Cu (1840–2000 mg/kg), Ba (1030–1150 mg/kg), Cr (315–445 mg/kg) and Ni (100–160 mg/kg). The weight loss on heating dried milled IBA to 1000 C ranged between 14.43 and 15.76%, suggesting that significant unburned organic carbon remained in this residue. The heterogeneity and highly hydrophilic character of IBA normally influence weight loss results. By 400 C the elemental carbon present in the bottom ash will start to pyrolyse, while entrapped water and bound water of hydration will escape by 550 C.
Fig. 4. X-ray diffraction data for different batches of milled bottom indicating nearly identical mineralogical composition.
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Table 2 Experimental data for sintered samples Fired density (g/cm3)
Shrinkage (%)
Water absorption (%)
Temperature ( C) 1080 Batch 1 Batch 2 Batch 3 Batch 4 Batch 5 Batch 6 a
1.27 1.28 1.30 1.30 1.28 1.32
1090 1.43 1.42 1.45 1.58 1.42 1.45
1100 1.68 1.73 1.96 1.91 1.61 1.69
1110 2.58 2.64 2.58 2.43 2.63 2.49
1115 2.40 2.33 2.34 2.16 2.29 2.23
1080 2.36 1.91 3.03 2.61 1.44 3.65
1090 4.79 5.06 5.91 6.17 5.44 7.47
1100 10.59 10.74 14.75 13.72 9.50 11.30
1110
1115
1080
1090
1100
1110
1115
21.66 21.69 21.89 20.85 21.01 21.33
a
42.71 42.55 41.24 41.58 42.23 40.52
34.37 34.78 33.64 26.97 34.99 33.68
24.24 22.83 15.46 16.33 26.23 23.97
1.90 0.76 0.91 0.56 1.38 0.50
0.85 0.97 0.54 0.52 0.52 0.66
a a a a a
No data due to sample deformation.
High weight loss during sintering compacted samples will cause increased porosity and reduced fired densities. The ’green’ densities of different batches of milled IBA pressed at 32 MPa ranged from 1.37 to 1.44 g/cm3. 3.2. Sintered properties 3.2.1. Fired density, shrinkage and water uptake Fired density, shrinkage and water uptake data are shown in Table 2. Maximum average densities between 2.43 and 2.64 g/cm3 were obtained by sintering at 1110 C. At higher temperatures lower density samples formed due to the formation of significant amounts of roughly spherical closed porosity. Sample shrinkage increases with firing temperature up to a maximum of 22% for samples fired at 1110 C. Above this temperature shrinkage measurements were not possible due to sample melting and deformation. Water absorption of sintered samples decreased rapidly as the firing temperature increases with values of between 0.5 and 1.9% obtained for samples fired at 1110 C.
Fig. 5. Micro-hardness indentation on a polished surface of sintered IBA.
3.2.2. Micro-hardness Some difficulties were experienced accurately measuring the micro-hardness of sintered IBA due to the relatively high level of porosity present on polished surfaces. Indentations close to pores can cause material to collapse making the indent dimensions difficult to measure accurately, as shown in the SEM micrograph in Fig. 5. The average micro-hardness obtained for sintered IBA was 247 kg mm2 (2.43 GPa). This is similar although slightly lower than values obtained for glassceramics derived from incinerator fly ash and fly ash containing glass matrix composites (Boccaccini et al., 2000a, b). It is likely that further optimisation and control of processing could reduce the porosity of sintered IBA and produce materials with improved physical properties. 3.2.3. Mineralogy and microstructure of sintered IBA XRD data of IBA sintered at 1110 C show a complex composition with overlapping peaks making unambiguous identification of the mineralogical phases present difficult. The main crystalline phases qualitatively identified in sintered samples of all IBA batches were wollastonite (CaSiO3) and diopside (CaMgSi2O6) with a 3+ minor presence of dorrite (Ca2Mg2Fe3+ )44 (Al,Fe Si2O20), clinoenstatite ((MgSiO3)2) and possibly albite (NaAlSi3O8). The crystalline phases detected in the milled and sintered IBA samples are compared in Table 3 and this clearly shows that sintering produces new crystalline phases not found in the original milled IBA. Wollastonite and diopside have previously been identified in glass–ceramics produced from raw materials containing IBA (Barbieri et al., 2000a, b; Romero et al., 1999). Wollastonite is rich in silicon and calcium and belongs to the pyroxenoid group of minerals. It can form from calcite and quartz at temperatures above 600–700 C and commonly occurs with calcite, tremolite, diopside, anorthite and a number of rare calcium-magnesium silicates. Diopside is an important rock-forming mineral that has also been identified as the major crystalline phase in glass–ceramics materials
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Fig. 6. (a) Secondary electron image of sintered IBA showing topographical features and (b) back-scattered electron image of the same area showing the distribution of different crystalline phases present due to differences in elemental composition.
produced from incinerator fly ash (Boccaccini et al., 1995). Fig. 6a and b show SEM micrographs of polished surfaces of sintered IBA ceramic produced by sintering batch 4 at 1100 C. Fig. 6a is a secondary electron image and this highlights topographical features. Fig. 6b is a backscattered electron image of the same area, and this shows the distribution of different crystalline phases present due to differences in elemental composition. The samples clearly contain significant amounts of approximately spherical pores within a silicate matrix of complex microstructure containing several different crystalline phases. The dark phase in backscattered images was found to be mainly composed of Si, while the bright regions were rich in Fe. The light background phase, which is a glassy matrix phase, was found to contain Si, Al, K, Ca, Ti and Fe. Fig. 7 shows the high porosity and intergranular nature of sintered IBA fracture surfaces.
3.3. Effect of sintering on the leaching characteristics of IBA 3.3.1. Acid neutralisation capacity Fig. 8 shows final leachate pH data as a function of acid addition from the ANC test. The results obtained were relatively consistent, with no significant differences observed between batches. Sintered ceramic materials produced from IBA exhibit a rapid decline in pH compared to the as-received bottom ash, corresponding to significantly reduced ANC. This change typically occurs on sintering and is due to solubilisation of alkali metal and alkali metal earth elements during milling, degradation of calcite to carbon dioxide and incorporation of CaO into the glassy and crystalline silicate structures (Van der Sloot et al., 2000; Johnson et al., 1995).
Table 3 Mineralogy of milled and sintered incinerator bottom ash Mineral
Milled
Quartz (SiO2)
++++
Calcite (CaCO3)
+++
Gehlenite (Ca2Al(AlSi)O7)
++
Hematite (Fe2O3)
++
Sintered (1110 C)
Wollastonite (CaSiO3)
++++
Diopside (CaMgSi2O6)
+++
Dorrite
3+ )4Si2O20) (Ca2Mg2Fe3+ 4 (Al,Fe
++
Clinoenstatite (MgSiO3)2
++
Albite (NaAlSi3O8)
+
++++, Major presence; +++, Medium presence; ++, Minor presence; +, Potential presence.
Fig. 7. Fracture surface of sintered IBA.
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Fig. 8. Acid neutralisation capacity data for as-received and sintered bottom ash.
3.3.2. Leachate analysis data The concentrations of selected alkali and alkali earth metals and heavy metals leached from both the asreceived IBA and the sintered ceramics produced from IBA are plotted as a function of final leachate pH in Fig. 9 (Ca, Na, Mg and K) and Fig. 10 (Al, Cu, Pb and Zn). Different batches showed very similar leaching
behaviour, with different metals having different characteristic release curves as a function of pH. Ceramic processing and sintering of IBA significantly reduced leaching of all elements apart from Na. Ca is reported to readily leach from as-received IBA due to the presence of CaCO3, CaSO4, CaCl2, CaO and Ca-silicates that are readily solubilised during the ANC test (Van der Sloot et al., 2000; Fa¨llman, 2000). Sintered IBA leaches significantly less Ca and Mg under all pH conditions. Na is extensively released over a wide pH range. However, there is significantly reduced Na leaching from sintered IBA ceramics under alkali conditions. Similar comments apply to leaching of K, where release under alkali conditions is again significantly reduced. The elements shown in Fig. 10 are the major heavy metals found to leach from IBA. At low pH conditions (< 3) the amounts released effectively represent the total available for leaching. Sintering reduces the availability of these metals and this is almost certainly due to encapsulation and incorporation into the glassy and crystalline phases formed. It is interesting to note that leaching also reduces under alkali leachate conditions. No significant Cu is released from any batch of sintered IBA samples at pH > 5.6 but it is consistently leached from the as-received IBA at 100–200 mg/kg over a pH range from 12 to 6. Aluminium leaches from as-received
Fig. 9. Leaching data for (a) Ca, (b) Na, (c) Mg and (d) K as a function of leachate pH for different batches of as-received and sintered IBA.
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Fig. 10. Leaching data for (a) Zn, (b) Pb, (c) Al and (d) Cu as a function of leachate pH for different batches of as-received and sintered IBA.
IBA under alkali conditions and this is reported to cause swelling and spalling problems when IBA is added to concrete (Pera et al., 1997; Pecqueur et al., 2001). Al is not leached from sintered IBA under alkali conditions and will only leach if the pH falls below 4.8.
4. Conclusions Ceramic processing involving wet milling, drying, pressing and sintering has been used to produce new ceramic materials from the < 8 mm fraction of IBA obtained from a modern EfW plant. The wet milling process produces a raw material for sintering that has consistent chemical composition with major crystalline phases of quartz (SiO2) and calcite (CaCO3), and minor presence of gehlenite (Ca2Al(AlSi)O7) and hematite (Fe2O3). Different milled batches have very similar mineralogy, indicating that it may be possible to produce a consistent raw material by processing IBA. All batches milled for 8 h had very similar particle size distributions. Maximum average densities between 2.43 and 2.64 g/cm3 were obtained by sintering milled IBA at 1110 C and these samples had water absorptions of between 0.5 and 1.9%. The samples exhibited spherical isolated pores within a complex silicate matrix microstructure characterised by a residual glassy phase and crystalline phases, as observed by SEM.
The main crystalline phases identified in sintered IBA ceramics were wollastonite (CaSiO3) and diopside (CaMgSi2O6) with a minor presence of dorrite (Ca2Mg23+ Fe3+ )4Si2O20), clinoenstatite (MgSiO3)2 and 4 (Al,Fe possibly albite (NaAlSi3O8). It is concluded that sintering produces new crystalline phases compared to those found in the original milled IBA. The average microhardness of sintered IBA was 2.43 GPa, which is lower than that found in similar sintered materials, mainly due to the relatively high porosity content. In order to improve the hardness and other mechanical properties of sintered IBA materials, a processing optimisation aiming at eliminating porosity should be carried out. Sintering significantly reduces ANC compared to asreceived IBA. The ceramics formed exhibit reduced leaching of Ca and Mg under all pH conditions. Leaching of Na and K at alkali pH levels is also significantly reduced. Sintering reduces the availability of heavy metals due to encapsulation and incorporation into glassy and crystalline phases. Cu and Al leaching is also significantly reduced under alkali conditions. The properties of sintered milled IBA derived materials are likely to be similar to clay-based ceramics. These could therefore potentially be used in a range of lowgrade applications such as industrial tiles for floors walls and roofs, linings for pipes and ducts in various applications and as aggregate, although clearly further process optimisation and material testing would be needed.
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Acknowledgements This work was completed as part of research funded by the UK Engineering and Physical Sciences Research Council (EPSRC) under the ‘Waste Minimisation through Recycling, Reuse and Recovery in Industry’ (WMR3) programme. We also wish to acknowledge the assistance of Richard Sweeney (Department of Materials) in completing the XRD analysis.
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capacity of municipal waste incinerator bottom ash. Environmental Science and Technology 29 (1), 142–147. Pavasars, I., 2000. Composition of organic matter in bottom ash from MSWI. In: Woolley, G.R., Goumans, J.J.J.M., Wainwright, P.J. (Eds.), Waste Materials in Construction. Elsevier Science, London, pp. 241–246. Pecqueur, G., Crignon, C., Que´ne´e, B., 2001. Behaviour of cementtreated MSWI bottom ash. Waste Management 21 (3), 229–233. Pera, J., Coutaz, L., Ambroise, J., Chababbet, M., 1997. Use of incinerator bottom ash in concrete. Cement and Concrete Research 27 (1), 1–5. Pfrang-Stotz, G., Reichelt, J., Roos, R., 2000. Chemical-mineralogical valuation of the leachate potential of municipal solid waste incineration (MSWI) bottom ashes. In: Woolley, G.R., Goumans, J.J.J.M., Wainwright, P.J. (Eds.), Waste Materials in Construction. Elsevier Science, London, pp. 975–984. Romero, M., Rawlings, R.D., Rinco´n, J.M., 1999. Development of a new glass-ceramic by means of controlled vitrification and crystallisation of inorganic wastes from urban incineration. Journal of the European Ceramic Society 19 (12), 2049–2058. Sawell, S.E., Chandler, A.J., Eighmy, T.T., Hartle´n, J., Hjelmar, O., Kosson, D., Van der Sloot, H.A., Vehlow, J., 1995. An international perspective on the characterisation and management of residues from MSW incinerators. Biomass and Bioenergy 9 (1-5), 377–386. Stegemann J.A., Coˆte´ P.L., 1991. Method 11: Acid Neutralisation Capacity, Test Methods for Solidified Waste Evaluation: a Cooperative Program (Report No. EPA 3/HA/8). Environment Canada, Ontario. Ullner, C., Germak, A., Le Doussal, H., Morrell, R., Reich, T., Vandermeulen, W., 2001. Hardness testing on advanced technical ceramics. Journal of the European Ceramic Society 21 (4), 439–451. Van der Sloot, H.A., Hoede, D., Cresswell, D.J.F., Barton, J.R., 2000. Leaching behaviour of synthetic aggregates. In: Woolley, G.R., Goumans, J.J.J.M., Wainwright, P.J. (Eds.), Waste Materials in Construction. Elsevier Science, London, pp. 695–706. Wainwright, P.J., 1981. Artificial aggregate from domestic refuse. Concrete 15 (5), 25–29. Wainwright, P.J., Boni, S.P.K., 1983. Some properties of concrete containing sintered domestic refuse as a coarse aggregate. Magazine of Concrete Research 35 (123), 75–85. Wainwright, P.J., Cresswell, D.J.F., 2001. Synthetic aggregates from combustion ashes using an innovative rotary kiln. Waste Management 21 (3), 241–246. Wang, K., Chiang, K., Perng, J., Sun, C., 1998. The characteristics study on sintering of municipal solid waste incinerator ashes. Journal of Hazardous Materials 59 (2–3), 201–210. Williams, P.T., 1998. Waste Treatment and Disposal. John Wiley and Sons, London. Woolley, G.R., Wainwright, P.J., Goumans, J.J.J.M., 2001. Science and engineering of recycling for environmental protection: an overview of the WASCON 2000 conference. Waste Management 21 (3), 211–212. Zevenbergen, C., Van der Wood, T., Bradley, J.P., Van der Broeck, P.F.C.W., Orbons, A.J., Van Reeuwijk, L.P., 1994. Morphological and chemical properties of MSWI bottom ash with respect to the glassy constituents. Hazardous Wastes and Hazardous Materials 11 (3), 371–383.
Ceramic processing of incinerator bottom ash C.R. Cheesemana,*, S. Monteiro da Rochaa, C. Sollarsa, S. Bethanisa, A.R. Boccaccinib a
Department of Civil and Environmental Engineering, Centre for Environmental Control and Waste Management, Imperial College of Science, Technology and Medicine, London SW7 2BU, UK b Department of Materials, Imperial College of Science, Technology and Medicine, London SW7 2BP, UK Accepted 25 February 2003
Abstract The < 8 mm fraction of aged incinerator bottom ash from a commercial incinerator (energy from waste) plant has been collected at regular intervals, characterised and processed to form ceramic materials. Ashes were sieved, wet ball milled, dried, compacted and sintered at temperatures between 1080 and 1115 C. Variations in the chemical composition and mineralogy of the milled ash, and the mineralogy, physical properties and leaching of sintered products have been assessed. Milling produces a raw material with consistent chemical and mineralogical composition with quartz (SiO2), calcite (CaCO3), gehlenite (Ca2Al(AlSi)O7) and hematite (Fe2O3) being the major crystalline phases present. Different batches also milled to give consistent particle size distributions. Sintering milled incinerator bottom ash at 1110 C produced ceramics with densities between 2.43 and 2.64 g/cm3 and major crystalline phases of wollastonite (CaSiO3) and diopside (CaMgSi2O6). The sintered ceramics had reduced acid neutralisation capacity compared to the as-received ash and exhibited reduced leaching of Ca, Mg, Na and K under all pH conditions. The leaching of heavy metals was also significantly reduced due to encapsulation and incorporation into glassy and crystalline phases, with Cu and Al showing greatly reduced leaching under alkali conditions. # 2003 Elsevier Ltd. All rights reserved.
1. Introduction Many countries increasingly consider incineration as an essential part of integrated management strategies for municipal solid waste (MSW). This is often driven by limited availability of landfill capacity and the advantages offered by incineration in modern ‘energy from waste’ (EfW) plants. Incineration reduces the volume of MSW by about 90%, but still produces signicant amounts of incinerator bottom ash (IBA) that need to be carefully managed. As MSW incineration and the resulting volumes of IBA increase, there is increasing need to develop new reuse applications for this material (Woolley et al., 2001; Chang et al., 1999). In England and Wales, approximately 28 million tonnes of MSW are currently produced each year, of which about 10% are currently incinerated in 14 operational EfW * Corresponding author. Tel.: +44-207-594-5971; fax: +44-207823-9401. E-mail address: [email protected] (C.R. Cheeseman). 0956-053X/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0956-053X(03)00039-4
plants. This produces in excess of 800 kilotonne of IBA, most of which is landfilled, although the potential for re-use of size sorted IBA in civil engineering applications is being extensively investigated. In some European countries such as The Netherlands, Denmark, Germany and France, processed bottom ash is routinely used in various construction projects (Wainwright, 1981; Wainwright and Boni, 1983; Sawell et al., 1995; Pera et al., 1997; Williams, 1998). IBA is a highly heterogeneous mix of slag, ferrous and non-ferrous metal, ceramics, glass, other non-combustibles and residual organic matter. The composition is directly related to the composition of the waste being incinerated and the sources of various elements in MSW are diverse and influence the characteristics of the IBA produced. For example, heavy metals such as cadmium, lead, mercury and zinc are mainly found in printing inks, paints, pigments, plastics and household batteries, while other wastes such as foil, cans, glass, ferrous and non-ferrous-materials may contain aluminium, chromium, copper, iron, nickel and alkaline-earth metals (Chimenos et al., 1999).
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At incinerator temperatures of 900–1000 C some elements volatilise and are collected in the air pollution control (APC) residues. The efficiency and type of incinerator influences partitioning and poor combustion can result in relatively high concentrations of volatile trace metals in IBA. The organic matter in IBA is mainly composed of un-combusted cellulose (74%) and lignin (20%) derived from the large amounts of paper, wood and other plant material found in MSW (Pavasars, 2000). Sintering is a high temperature process that produces strength and other engineering properties in compacted fine powdered materials through particle bonding and densification. It is widely used in industry to produce a diversity of materials, including ceramics, metals and composites. Sintering has the potential to convert a range of inorganic waste ashes such as MSW IBA into environmentally benign new solid-monolithic ceramic products with properties that may allow reuse in appropriate applications (Boccaccini et al., 2000a). The inherent variability of IBA means that processing prior to sintering is essential if consistent ceramic materials are to be produced. Although the effects of sintering incinerator fly ash have been investigated, the properties of sintered IBA have not been widely reported (Wang et al., 1998; Wainwright and Cresswell, 2001; Bethanis et al., 2002). IBA has been used as a raw material combined with glass cullet for the production of glass–ceramics, and glass–ceramics have been formed from mixes containing IBA, glass and an industrial waste fly ash (Barbieri et al., 2000a, b). Glass matrix composites have also been produced from IBA combined with aluminium foundry waste (Ferraris et al., 2001). The glass–ceramic production process involves high temperature melting (vitrification) combined with subsequent heat treatment to induce devitrification. The process investigated in this work is different, produces different materials and involves direct sintering of processed IBA at temperatures significantly below those needed for vitrification. The main objectives of this work were to investigate the properties of materials made by applying ceramic processing to IBA and to assess the effects of IBA variability. Materials obtained at different stages of the process have been characterised and the leaching of as-received IBA and sintered IBA compared.
mass burn incineration technology with controlled high temperature combustion combined with advanced fluegas cleaning. The IBA produced is transported off-site, weathered for 6–8 weeks and the readily extractable ferrous and non-ferrous metals removed for recycling. The remaining material is screened into different sizes as the coarse fractions have potential uses as aggregate and inert fill. There are currently no viable re-use applications specifically for the < 8 mm fraction of IBA although this represents approximately 45 wt.% of the total ash. Representative samples of weathered IBA from which readily extractable metals had been removed were collected over a 10-week period during 2001. Each of the six batches collected was screened through an 8 mm sieve and the material passing used throughout subsequent experiments. Variability in the particle size distribution of the < 8 mm IBA samples was determined by sieve analysis using 150, 355 mm, 1.70 and 3.35 mm ASTM standard sieves. 2.1.2. Production of milled bottom ash Each batch of < 8 mm IBA was oven dried overnight at 105 C and wet ball milled for 8 h in a 3-l polypropylene mill, rotating at 50 rpm, using high-density alumina media. 500 g batches of ash were milled at a water-to-ash ratio of 2. Slurries obtained from milling were passed through 1.70 mm and 355-mm sieves to remove coarse particles and the coarse material retained by the 1.70-mm sieve was dried and weighed. The < 355 mm fraction was de-watered by pressure filtration in a stainless steel extraction vessel on Whatman GF/C filter paper. The filter cake obtained was oven dried overnight at 105 C, ground in a pestle and mortar and passed through a 150-mm sieve to produce a fine homogeneous grey powder suitable for sintering. 2.1.3. Production of sintered ceramic samples The dried milled powder was uni-axially compacted at 32 MPa in a stainless steel die to form cylindrical ‘green’ samples, approximately 20 mm in diameter and 12–15 mm in height, without the addition of an organic binder or other additives. These were sintered in an electric furnace at temperatures between 1080 and 1120 C using a ramp rate of 6 C min1 with a dwell time of 60 min at the maximum temperature. 2.2. Materials characterisation
2. Materials and methods 2.1. Materials processing 2.1.1. Sampling MSW IBA from a major EfW plant situated in southeast England has been used in this work. This plant processes approximately 420 000 tonnes of waste per year and generates 35 MWh of electricity. It uses well-established
2.2.1. Characterisation of milled IBA The particle size distribution between 0.4 and 900 mm in each batch of milled slurry was analysed by laser diffraction analysis (Beckman Coulter LS-100). 2.2.2. X-Ray powder diffraction The crystalline phases present in the dried milled ash batches were qualitatively identified by X-ray diffraction
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(XRD) using a Philips PW1830 diffractometer system fitted with a PW1820 goniometer, an automatic divergence slit and a graphite monochromator with CuKa radiation at an accelerating voltage of 40 kV. 2.2.3. Chemical characterisation The elemental composition of dried milled bottom ash was determined by digestion using lithium metaborate and tetraborate flux fusion (Ingamells, 1970). Digests were analysed by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The weight loss was determined by heating 4-g samples to 1000 C for 12 h. This includes the weight loss due to combustion of organic matter, dehydration and the degradation of carbonated species formed by atmospheric exposure. 2.2.4. Characterisation of sintered IBA samples The density of sintered samples was determined using Archimedes’ method and the linear shrinkage calculated by comparing sample diameters before and after firing. Water absorption of sintered IBA was determined from the weight increase of surface dry samples after immersion in water for 24 h (BS 812, 1975). Samples of sintered IBA were ground to < 150mm and analysed by XRD to identify the crystalline phases present. The methodology and equipment used was the same as for XRD analysis of milled bottom ash samples. Scanning electron microscopy (SEM, Jeol JSM-T200) was used to examine the microstructure of carbon coated polished surfaces and fracture surfaces of IBA samples sintered at 1100 C. Samples were also analysed for elements present in different phases by energy dispersive X-ray (EDX) spectroscopy. The Vickers micro-hardness of IBA samples sintered at 1100 C was measured using a Leitz Wetzlar 8423 microhardness tester with 25 g load. Prior to hardness testing the samples were embedded in epoxy resin and polished to a 0.5 mm diamond finish. Average hardness values were calculated from five indents on sintered samples from batches 1, 4 and 5. The Vickers micro-hardness test uses a square-based diamond pyramid with opposite sides meeting at the apex at an angle of 136 . The diamond is pressed into the surface of the material and the size of the impression measured using a calibrated microscope. The Vickers number (HV), given in units of kg mm2, is calculated from: HV ¼ 1:854 F=D 2 ;
where F is the applied load (kgf) and D2 the area of the indentation measured in mm2 (Ullner et al., 2001).
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2.3. Metal leaching from as-received and sintered IBA The acid neutralisation capacity (ANC) test, combined with leachate analysis, was completed on all batches of as-received IBA and IBA sintered at 1100 C (Stegemann and Coˆte´, 1991). This test allows the leaching under different pH conditions to be assessed. Samples for leaching were dried and ground to pass through a 150-mm sieve and 5-g samples mixed with 30 ml of aqueous acid solutions that varied in concentration between 0 and 100% 2.0 N nitric acid over 11 equal increments. The slurries obtained were mixed for 48 h in a rotary extractor and then centrifuged at 6000 rpm for 10 min to separate the leachate. The leachant/ sample contact time of 48 h allows the solids to equilibrate and centrifugation reduces interference in pH analysis. The leachate was extracted and filtered through a 0.45-mm membrane filter and acidified with 10% volume HNO3 prior to analysis by ICP–AES for a range of elements including Co, Cd, Cr, Cu, Fe, Mn, Ni, Pb, and Zn.
3. Results and discussion 3.1. As-received and milled IBA characterisation The sieve analysis data for the different batches of < 8 mm IBA is shown in Fig. 1. This indicates fairly consistent results at size fractions smaller than 355 mm, while at larger particle sizes significant variability occurs, with Batch 5 exhibiting lower content of coarse particles than the other batches. After milling for 8 h the slurry was passed through 1.70mm and 355-mm sieves and the residue collected on the sieves washed and dried. The amount greater than 1.7 mm
Fig. 1. Particle size distribution of the as-received batches of <8 mm incinerator bottom ash.
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Fig. 2. Percentage weight of coarse material retained on a 1.70-mm sieve after 8 h wet ball milling of <8 mm IBA.
is shown for each batch in Fig. 2. This typically represents about 30% of the original < 8 mm fraction and mainly consists of rounded glass and other difficult to mill materials, such as rock, bone, wood, some plastic and metal. It is a processing by-product that has potential to be used after further screening as normal weight aggregate. The particle size distribution of the different IBA batches after milling for 8 h is shown in Fig. 3. All batches show very similar distributions, with typically 50% of the particles with diameter finer than 8 mm and 95% less than 50 mm. Particle size distribution is an important factor in ceramics production as this has a key role in controlling sintering. It is expected that the 8-h milling time required to produce this type of particle size distribution could be significantly reduced using larger, commercial scale ball milling equipment.
XRD data for different milled IBA batches is shown in Fig. 4. This shows that the major crystalline phases qualitatively identified in the different milled batches are very similar, indicating that it may be possible to produce a consistent raw material by processing IBA. The major phases identified are quartz (SiO2) and calcite (CaCO3), with a minor presence of gehlenite (Ca2Al(AlSi)O7) and Table 1 Chemical composition of different batches of milled incinerator bottom ash Component Batch 1 Batch 2 Batch 3 Batch 4 Batch 5 Batch 6 Major elements (wt.%) SiO2 34.29 Al2O3 10.83 Fe2O3 7.38 MgO 2.16 CaO 20.80 Na2O 2.42 K2O 0.83 1.58 P2O5 TiO2 1.24 MnO 0.11
34.27 10.89 7.46 2.08 21.21 2.16 0.82 1.58 1.34 0.10
34.91 10.45 7.90 2.19 21.34 2.40 0.81 1.65 1.34 0.11
35.61 10.28 7.77 2.04 20.78 2.36 0.83 1.60 1.25 0.13
35.52 10.50 7.87 2.13 20.47 2.45 0.85 1.60 1.29 0.11
34.31 10.78 7.69 2.09 20.52 2.32 0.82 1.56 1.23 0.11
LOIa
15.76
14.62
14.88
14.43
15.28
15.01
Minor and trace elements content (mg/kg) Ba 1045 1030 1150 Be 0.80 1.2 1.4 Co 20 20 25 Cr 445 340 355 Cu 2000 2000 1900 Ni 160 100 110 Sc 2 4 4 Sr 410 350 350 V 45 35 35 Y 6 8 10 Zn 2800 4000 2500 Zr 160 175 180 Fig. 3. Particle size distribution of batches of <8 mm IBA after milling for 8 h.
a
LOI, loss on ignition.
1120 1.2 20 390 2150 145 4 350 35 8 3300 175
1060 1.4 20 355 1840 120 4 350 35 10 2700 180
1030 1.2 20 315 1950 110 4 360 30 8 3300 175
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hematite (Fe2O3) and these minerals have previously been identified as major components of incinerator bottom ash (Pfrang-Stotz et al., 2000; Zevenbergen et al., 1994). Chemical composition data for the different batches of milled IBA is shown in Table 1. All milled batches show very consistent composition in terms of major oxides, with approximately 70% of the weight consisting of silica, alumina, iron and calcium oxides. The concentrations of trace elements are also very similar, with the exception of Zn, which shows relatively high variability (2500–4000 mg/kg). Heavy
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metals of particular environmental concern present at high concentrations include Cu (1840–2000 mg/kg), Ba (1030–1150 mg/kg), Cr (315–445 mg/kg) and Ni (100–160 mg/kg). The weight loss on heating dried milled IBA to 1000 C ranged between 14.43 and 15.76%, suggesting that significant unburned organic carbon remained in this residue. The heterogeneity and highly hydrophilic character of IBA normally influence weight loss results. By 400 C the elemental carbon present in the bottom ash will start to pyrolyse, while entrapped water and bound water of hydration will escape by 550 C.
Fig. 4. X-ray diffraction data for different batches of milled bottom indicating nearly identical mineralogical composition.
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Table 2 Experimental data for sintered samples Fired density (g/cm3)
Shrinkage (%)
Water absorption (%)
Temperature ( C) 1080 Batch 1 Batch 2 Batch 3 Batch 4 Batch 5 Batch 6 a
1.27 1.28 1.30 1.30 1.28 1.32
1090 1.43 1.42 1.45 1.58 1.42 1.45
1100 1.68 1.73 1.96 1.91 1.61 1.69
1110 2.58 2.64 2.58 2.43 2.63 2.49
1115 2.40 2.33 2.34 2.16 2.29 2.23
1080 2.36 1.91 3.03 2.61 1.44 3.65
1090 4.79 5.06 5.91 6.17 5.44 7.47
1100 10.59 10.74 14.75 13.72 9.50 11.30
1110
1115
1080
1090
1100
1110
1115
21.66 21.69 21.89 20.85 21.01 21.33
a
42.71 42.55 41.24 41.58 42.23 40.52
34.37 34.78 33.64 26.97 34.99 33.68
24.24 22.83 15.46 16.33 26.23 23.97
1.90 0.76 0.91 0.56 1.38 0.50
0.85 0.97 0.54 0.52 0.52 0.66
a a a a a
No data due to sample deformation.
High weight loss during sintering compacted samples will cause increased porosity and reduced fired densities. The ’green’ densities of different batches of milled IBA pressed at 32 MPa ranged from 1.37 to 1.44 g/cm3. 3.2. Sintered properties 3.2.1. Fired density, shrinkage and water uptake Fired density, shrinkage and water uptake data are shown in Table 2. Maximum average densities between 2.43 and 2.64 g/cm3 were obtained by sintering at 1110 C. At higher temperatures lower density samples formed due to the formation of significant amounts of roughly spherical closed porosity. Sample shrinkage increases with firing temperature up to a maximum of 22% for samples fired at 1110 C. Above this temperature shrinkage measurements were not possible due to sample melting and deformation. Water absorption of sintered samples decreased rapidly as the firing temperature increases with values of between 0.5 and 1.9% obtained for samples fired at 1110 C.
Fig. 5. Micro-hardness indentation on a polished surface of sintered IBA.
3.2.2. Micro-hardness Some difficulties were experienced accurately measuring the micro-hardness of sintered IBA due to the relatively high level of porosity present on polished surfaces. Indentations close to pores can cause material to collapse making the indent dimensions difficult to measure accurately, as shown in the SEM micrograph in Fig. 5. The average micro-hardness obtained for sintered IBA was 247 kg mm2 (2.43 GPa). This is similar although slightly lower than values obtained for glassceramics derived from incinerator fly ash and fly ash containing glass matrix composites (Boccaccini et al., 2000a, b). It is likely that further optimisation and control of processing could reduce the porosity of sintered IBA and produce materials with improved physical properties. 3.2.3. Mineralogy and microstructure of sintered IBA XRD data of IBA sintered at 1110 C show a complex composition with overlapping peaks making unambiguous identification of the mineralogical phases present difficult. The main crystalline phases qualitatively identified in sintered samples of all IBA batches were wollastonite (CaSiO3) and diopside (CaMgSi2O6) with a 3+ minor presence of dorrite (Ca2Mg2Fe3+ )44 (Al,Fe Si2O20), clinoenstatite ((MgSiO3)2) and possibly albite (NaAlSi3O8). The crystalline phases detected in the milled and sintered IBA samples are compared in Table 3 and this clearly shows that sintering produces new crystalline phases not found in the original milled IBA. Wollastonite and diopside have previously been identified in glass–ceramics produced from raw materials containing IBA (Barbieri et al., 2000a, b; Romero et al., 1999). Wollastonite is rich in silicon and calcium and belongs to the pyroxenoid group of minerals. It can form from calcite and quartz at temperatures above 600–700 C and commonly occurs with calcite, tremolite, diopside, anorthite and a number of rare calcium-magnesium silicates. Diopside is an important rock-forming mineral that has also been identified as the major crystalline phase in glass–ceramics materials
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Fig. 6. (a) Secondary electron image of sintered IBA showing topographical features and (b) back-scattered electron image of the same area showing the distribution of different crystalline phases present due to differences in elemental composition.
produced from incinerator fly ash (Boccaccini et al., 1995). Fig. 6a and b show SEM micrographs of polished surfaces of sintered IBA ceramic produced by sintering batch 4 at 1100 C. Fig. 6a is a secondary electron image and this highlights topographical features. Fig. 6b is a backscattered electron image of the same area, and this shows the distribution of different crystalline phases present due to differences in elemental composition. The samples clearly contain significant amounts of approximately spherical pores within a silicate matrix of complex microstructure containing several different crystalline phases. The dark phase in backscattered images was found to be mainly composed of Si, while the bright regions were rich in Fe. The light background phase, which is a glassy matrix phase, was found to contain Si, Al, K, Ca, Ti and Fe. Fig. 7 shows the high porosity and intergranular nature of sintered IBA fracture surfaces.
3.3. Effect of sintering on the leaching characteristics of IBA 3.3.1. Acid neutralisation capacity Fig. 8 shows final leachate pH data as a function of acid addition from the ANC test. The results obtained were relatively consistent, with no significant differences observed between batches. Sintered ceramic materials produced from IBA exhibit a rapid decline in pH compared to the as-received bottom ash, corresponding to significantly reduced ANC. This change typically occurs on sintering and is due to solubilisation of alkali metal and alkali metal earth elements during milling, degradation of calcite to carbon dioxide and incorporation of CaO into the glassy and crystalline silicate structures (Van der Sloot et al., 2000; Johnson et al., 1995).
Table 3 Mineralogy of milled and sintered incinerator bottom ash Mineral
Milled
Quartz (SiO2)
++++
Calcite (CaCO3)
+++
Gehlenite (Ca2Al(AlSi)O7)
++
Hematite (Fe2O3)
++
Sintered (1110 C)
Wollastonite (CaSiO3)
++++
Diopside (CaMgSi2O6)
+++
Dorrite
3+ )4Si2O20) (Ca2Mg2Fe3+ 4 (Al,Fe
++
Clinoenstatite (MgSiO3)2
++
Albite (NaAlSi3O8)
+
++++, Major presence; +++, Medium presence; ++, Minor presence; +, Potential presence.
Fig. 7. Fracture surface of sintered IBA.
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Fig. 8. Acid neutralisation capacity data for as-received and sintered bottom ash.
3.3.2. Leachate analysis data The concentrations of selected alkali and alkali earth metals and heavy metals leached from both the asreceived IBA and the sintered ceramics produced from IBA are plotted as a function of final leachate pH in Fig. 9 (Ca, Na, Mg and K) and Fig. 10 (Al, Cu, Pb and Zn). Different batches showed very similar leaching
behaviour, with different metals having different characteristic release curves as a function of pH. Ceramic processing and sintering of IBA significantly reduced leaching of all elements apart from Na. Ca is reported to readily leach from as-received IBA due to the presence of CaCO3, CaSO4, CaCl2, CaO and Ca-silicates that are readily solubilised during the ANC test (Van der Sloot et al., 2000; Fa¨llman, 2000). Sintered IBA leaches significantly less Ca and Mg under all pH conditions. Na is extensively released over a wide pH range. However, there is significantly reduced Na leaching from sintered IBA ceramics under alkali conditions. Similar comments apply to leaching of K, where release under alkali conditions is again significantly reduced. The elements shown in Fig. 10 are the major heavy metals found to leach from IBA. At low pH conditions (< 3) the amounts released effectively represent the total available for leaching. Sintering reduces the availability of these metals and this is almost certainly due to encapsulation and incorporation into the glassy and crystalline phases formed. It is interesting to note that leaching also reduces under alkali leachate conditions. No significant Cu is released from any batch of sintered IBA samples at pH > 5.6 but it is consistently leached from the as-received IBA at 100–200 mg/kg over a pH range from 12 to 6. Aluminium leaches from as-received
Fig. 9. Leaching data for (a) Ca, (b) Na, (c) Mg and (d) K as a function of leachate pH for different batches of as-received and sintered IBA.
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Fig. 10. Leaching data for (a) Zn, (b) Pb, (c) Al and (d) Cu as a function of leachate pH for different batches of as-received and sintered IBA.
IBA under alkali conditions and this is reported to cause swelling and spalling problems when IBA is added to concrete (Pera et al., 1997; Pecqueur et al., 2001). Al is not leached from sintered IBA under alkali conditions and will only leach if the pH falls below 4.8.
4. Conclusions Ceramic processing involving wet milling, drying, pressing and sintering has been used to produce new ceramic materials from the < 8 mm fraction of IBA obtained from a modern EfW plant. The wet milling process produces a raw material for sintering that has consistent chemical composition with major crystalline phases of quartz (SiO2) and calcite (CaCO3), and minor presence of gehlenite (Ca2Al(AlSi)O7) and hematite (Fe2O3). Different milled batches have very similar mineralogy, indicating that it may be possible to produce a consistent raw material by processing IBA. All batches milled for 8 h had very similar particle size distributions. Maximum average densities between 2.43 and 2.64 g/cm3 were obtained by sintering milled IBA at 1110 C and these samples had water absorptions of between 0.5 and 1.9%. The samples exhibited spherical isolated pores within a complex silicate matrix microstructure characterised by a residual glassy phase and crystalline phases, as observed by SEM.
The main crystalline phases identified in sintered IBA ceramics were wollastonite (CaSiO3) and diopside (CaMgSi2O6) with a minor presence of dorrite (Ca2Mg23+ Fe3+ )4Si2O20), clinoenstatite (MgSiO3)2 and 4 (Al,Fe possibly albite (NaAlSi3O8). It is concluded that sintering produces new crystalline phases compared to those found in the original milled IBA. The average microhardness of sintered IBA was 2.43 GPa, which is lower than that found in similar sintered materials, mainly due to the relatively high porosity content. In order to improve the hardness and other mechanical properties of sintered IBA materials, a processing optimisation aiming at eliminating porosity should be carried out. Sintering significantly reduces ANC compared to asreceived IBA. The ceramics formed exhibit reduced leaching of Ca and Mg under all pH conditions. Leaching of Na and K at alkali pH levels is also significantly reduced. Sintering reduces the availability of heavy metals due to encapsulation and incorporation into glassy and crystalline phases. Cu and Al leaching is also significantly reduced under alkali conditions. The properties of sintered milled IBA derived materials are likely to be similar to clay-based ceramics. These could therefore potentially be used in a range of lowgrade applications such as industrial tiles for floors walls and roofs, linings for pipes and ducts in various applications and as aggregate, although clearly further process optimisation and material testing would be needed.
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Acknowledgements This work was completed as part of research funded by the UK Engineering and Physical Sciences Research Council (EPSRC) under the ‘Waste Minimisation through Recycling, Reuse and Recovery in Industry’ (WMR3) programme. We also wish to acknowledge the assistance of Richard Sweeney (Department of Materials) in completing the XRD analysis.
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