Properties and Leaching of Incinerator Bottom Ash

Properties and Leaching of Incinerator Bottom Ash

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Effect of Sintering Temperature on the Properties and Leaching of Incinerator Bottom Ash

ABSTRACT

The fine particle size fraction of municipal solid waste incinerator bottom ash is often problematic because reuse applications for this material are limited. In these experiments incinerator bottom ash with a particle size of less than 8 mm has been processed using conventional ceramic production techniques involving wet milling, drying, compacting and sintering. The effect of sintering temperature on the sintered density, microstructure, acid neutralisation capacity (ANC) and the release of metal ions as a function of leachate pH are reported. Sintering at 1080?C produced samples with maximum density. This material contained diopside (CaMgSi2O6), clinoenstatite (Mg2Si2O6) and wollastonite (CaSiO3) as the major crystalline phases. The acid neutralization capacity of sintered samples is significantly lower than milled bottom ash, and further reduces as the sintering temperature increases. This is associated with reduced leaching of Ca from sintered ash samples under all leachate pH conditions. Heavy metals present in the incinerator bottom ash included Cr, Cu, Ni, Zn, Cd and Pb. Sintering under optimum conditions reduced the leachable fraction of these metals under aggressive acid conditions (leachate pH 3) by factors ranging from 90% for Ni to greater than 99% for Cr, Cd, Zn and Pb.

1. INTRODUCTION

The role of waste incineration in integrated waste management continues to increase in many countries, and particularly those where the population density is high and the availability of landfill is limited. Incineration has the advantage that the energy released can be used to generate power and/or provide heating to the local community and the emissions to atmosphere from modern 'energy from waste' (EfW) plants are now strictly controlled. Although incineration reduces the volume of municipal waste by up to 80-90%, EfW plants still produce considerable amounts of incinerator bottom ash (IBA) that is currently either disposed of in landfill or reused in relatively low-grade civil engineering applications (Wiles & Shepherd 1999).

Developments in solid waste management in industrialised countries are increasingly aimed at reducing the amount of waste requiring landfill by using these materials as resources (Woolley et al. 2001, Chang et al. 1999). IBA is a heterogeneous mix of ceramic materials such as brick, stone and glass, ferrous and non-ferrous metals and other non-combustible and residual organic matter (Cheminos et al. 1999, Wiles, 1996, Zevenbergen et al. 1994). In 2000, the 11 operating EfW plants in England processed 8% of the 27.6 million tonnes of municipal solid waste (MSW) generated. This resulted in 2.77 million tonnes of solid residues, 79% of which went direct to landfill, while the remainder was processed prior to use as bulk fill in applications such as embankments or as a substitute normal weight aggregate in asphalt or construction blocks.

The use of IBA in concrete and as a road base material has previously been investigated (Pera et al. 1997, Berg & Neal, 1998, Quenee et al. 2000, Schreurs et al. 2000, Bruder-Habscher et al. 2001). By screening IBA a number of different grades can be separated and the coarser fractions can be used as bulk fill for road building and as drainage media. Finer material is more problematic, although it is being used as a protection layer for landfill liner membranes and has potential use in the production of lightweight concrete building blocks. The fine IBA fraction can be problematic because of compositional variability and because of the adverse effect constituents of IBA may have on concrete. There are also concerns over leaching of heavy metals and therefore there may be advantages in further processing the fine fraction of IBA to produce new, more homogenous materials to enable this potential resource to be beneficially reused. This would have benefits both in terms of resource recovery and reduced dependence on the extraction of natural materials.

High temperature sintering can convert inorganic wastes into solid-monolithic ceramic products with a range of different microstructural characteristics and properties. This type of processing is also reported to alter waste component leaching (Selinger & Schmidt 1997, Wang et al. 1998, Wang et al. 2001). IBA has previously been combined with glass cullet, incinerator fly ash and filter dusts to form glass-ceramics (Ferraris et al. 2001, Barbieri et al. 2000a, Barbieri et al. 2000b, Romero et al. 2001). These processes have generally involved heating mixed wastes to high temperatures (>1300oC) to cause vitrification, followed by heat treatment to induce controlled crystallisation effects.

The process investigated in this work has involved sintering milled IBA at much lower temperatures than required for vitrification, and this is expected to produce different materials from those reported in previous studies. The aim was to characterise the ceramics produced by compacting and sintering milled IBA at different temperatures, and to report the effect of sintering on the acid neutralisation capacity (ANC) and metal leaching characteristics.

2. EXPERIMENTAL WORK

2.1 Material and processing

IBA was obtained from a major EfW plant situated in SE England with the capacity to burn 420,000 tonnes per year of municipal solid waste. This plant uses conventional mass-burn technology and generates 34 MWh of electricity.

The IBA used in these experiments had been weathered for between six to eight weeks and the ferrous and non-ferrous metals removed by magnetic separation. The remaining ash was sorted into different sizes and the ash fraction with a particle size less than 8mm collected. This fraction represents approximately 45% w/w of the total weathered IBA and is the part for which commercially viable reuse applications have so far been relatively difficult to identify. The samples used in these experiments had been stored in the laboratory for up to 1 year and were therefore aged and extensively carbonated.

500g sub-samples of IBA were wet milled at a water/solids ratio of 2, in a 3 litre polypropylene mill rotating at 50 rpm, using high-density alumina milling media. The ash was milled for 2, 8 and 16 hours and the particle-size distribution determined by laser diffraction (Beckman Coulter, LS-100). Milled slurries were passed through a 355?m sieve to remove oversize, predominantly glassy material, and de-watered by pressure filtration on Whatman GF/C filter paper using a stainless steel extraction vessel. The filter cakes formed were oven-dried overnight at 105oC and lightly ground in a pestle and mortar to produce a powder suitable for subsequent processing.

2.2 Milled ash characterisation

The composition of milled IBA was determined by fusion with lithium metaborate and lithium tetraborate flux to 1000oC and acid dissolution (Ingamells 1970). The digests were analysed using inductively coupled plasma atomic emission spectroscopy (ICP-AES, Perkin Elmer Optima). The Optima ICP-AES has the capacity to use multiple emission lines simultaneously and has increased capability to resolve inter-element interference and background matrix effects.

Crystalline phases present in milled ash were determined by X-ray diffraction (XRD Phillips PW1810 diffractometer system) using CuK? radiation at an accelerating voltage of 40 Kv.

2.3 Production of sintered samples

Dried milled bottom ash was uniaxially pressed at 32 MPa in a stainless steel compaction die, to form 20mm diameter and approximately 22mm high cylinders, without the addition of any organic binder. The pressed samples were heated in an electric fired chamber furnace to temperatures between 1020 and 1100oC using a ramp rate of 6oCmin-1 and a 1 hour dwell at the maximum temperature.

2.4 Characterisation of sintered specimens

Sintered sample density was measured using the Archimedes method. The water absorption was determined from the increase in weight of 'surface dry' sintered samples after submersion in water for 24 hours and indicates the extent of surface connected water accessible porosity in the material. The microstructure of sintered samples was investigated by examining surfaces polished using 1?m diamond paste, using scanning electron microscopy (SEM- Phillips XL40).

Sintered samples were crushed and ground to a particle size less than 150?m to determine the crystalline phases present by XRD. Ground sintered samples were also digested in duplicate and analysed for chemical composition. In both cases the methods were the same as used for characterising milled IBA.

2.5 Leach testing

The effect of sintering on the pH dependent leaching behaviour of heavy metals was investigated using the acid neutralisation capacity (ANC) test (Stegemann & Cot? 1991). Samples of 16 hour-milled IBA and milled IBA sintered at 1020, 1080 and 1100?C were ground to a particle size less than 150?m. A series of 5g sub-samples were mixed with 30 ml solutions of varying acidity, ranging from distilled water to 2.0N HNO3 over 11 equal increments. The samples and leachant were mixed for 48 hours in sealed containers on a rotary extractor, before being centrifuged to separate the leachate. The leachate pH was measured and the leachate extracted by filtering through a 0.45?m membrane filter and acidified with 10% volume HNO3 prior to metal analysis by ICP-AES.

3. RESULTS

3.1 Particle size distribution data

The effect of milling IBA for 2, 8 and 16 hours on the particle size distribution, after removal of oversize material (> 355 ?m), is shown in Figure 1. Fifty percent of the volume (d50 value) of the as-received ash was less than 295?m, and this decreased to 15?m after 2 hours milling. Corresponding d50 values for 8 and 16 hour-milled ash were 6.2 and 3.9?m.

3.2 Physical properties of sintered ash samples

The effect of firing temperature on the sintered density of IBA is shown in Figure 2. Milled ash particle size is a key parameter controlling the sintering, with fine particles producing higher densities. Increasing the sintering temperature to 1080oC for 8 and 16 hour-milled samples increased density to a maximum of ~2.6 g/cm3. Firing above 1080oC caused a decrease in density, associated with sample expansion and bloating. Milling IBA for 2 hours produced sintered samples with significantly reduced densities.

Figure 3 shows the effect of sintering on water absorption. Increasing firing temperature decreased water absorption with milled ash samples becoming almost impermeable when sintered at or above 1080oC. Sintering had only a marginal effect on the water absorption of the 2 hour-milled samples.

3.3 Chemical composition of milled and sintered IBA

The average chemical composition data for milled and sintered IBA is shown in Table 1. The major elements in IBA are Si, Ca, Al and Fe. Heavy metals of environmental concern that were present at relatively high concentrations included Zn, Pb, Cu, Cr and Ni and the concentrations are within the range reported in other studies of IBA composition. It should be noted that the milled IBA is expected to contain significant levels of carbonated species and residual organic matter that breakdown during heating to the sintering temperature. Comparing the chemical composition of these materials with sintered samples, bearing in mind the inherent variability in IBA composition indicates that sintering at these temperatures is not associated with any significant changes in the concentration of trace elements.

3.4 Crystalline phases present in milled and sintered IBA

XRD data for the milled ash is shown in Figure 4. This was found to contain quartz (SiO2), calcite (CaCO3), ghelenite (Ca2Al2SiO7) and hematite (Fe2O3) in general agreement with the mineralogy of IBA previously reported (Kirkby & Rimstidt 1993, Dykstra et al. 1999).

XRD data for milled IBA sintered at 1020, 1080 and 1100oC is shown in Figure 5 and the results summarised in Table 2. Milled IBA sintered at 1020oC contained diopside (CaMgSi2O6) as the principal crystalline phase, together with significant amounts of wollastonite (CaSiO3), and minor amounts of albite (NaAlSi3O8) and hematite. Quartz and calcite peaks were not visible in the XRD data of IBA sintered at 1020oC.

IBA sintered at 1080oC had similar mineralogy to ash sintered at 1020oC. Diopside was the principal crystalline phase together with wollastonite and clinoenstatite (Mg2Si2O6). Albite and hematite were also present in smaller amounts.

XRD analysis of IBA sintered at 1100oC showed that augite (Ca,Na) (Mg,Fe,Al,Ti) (Si,Al)2O6 was the major crystalline phase, with anorthite (CaAl2Si2O8) and wollastonite as secondary crystalline phases. The data also indicates the presence of a small amount of albite.

3.5 Microstructural analysis of sintered samples

SEM micrographs of polished surfaces of IBA sintered at 1020, 1080 and 1100oC are shown in Figure 6. The sample sintered at 1020oC was not extensively sintered and had a high level of porosity, as reflected by the low density and high water absorption determined for these samples. The surface of samples sintered to the maximum density at 1080oC showed a relatively dense, sintered microstructure containing mainly isolated porosity. Samples sintered at 1100oC had reduced densities and this is clearly associated with the formation of approximately spherical closed pores. This is believed to be due to evolution of SO2 from the decomposition of metal sulphates, together with softening of the glassy material present in the ash (Stern & Weise 1966, Bethanis et al. 2002).

3.6 Acid Neutralisation Capacity (ANC)

ANC data for the milled IBA, and IBA sintered at 1020, 1080 and 1100oC is shown in Figure 7. ANC is primarily controlled by the solubility of calcium minerals and other alkali metals and alkali metal earth elements, and the resulting leachate pH strongly influences contaminant metal solubility and leaching (Johnson 1995, Meima & Comans 1997).

In distilled water (0 eq/Kg) the milled IBA resulted in a leachate pH of 8.7. This relatively low pH is due to the virtual absence of soluble basic calcium hydroxide/silicate species, due to extensive carbonation and neutralisation resulting in the formation of less soluble Ca-containing minerals. CaCO3 is present in IBA as a result of carbonation during ageing and this is reported to provide buffering capacity to acid addition in the pH range between 5 and 6, due to the formation of Ca2+, HCO3- and OH- ions (Kirkby & Rimstidt 1994).

While the milled IBA has significant ANC between pH 5 and 6, sintered IBA ceramics exhibit a rapid decline in leachate pH with acid addition through this pH range, indicating reduced ANC. This is due to decomposition of CaCO3 to CaO and CO2 that occurs during heating to the sintering temperature, combined with subsequent encapsulation and incorporation of Ca2+ into both amorphous and crystalline phases present in the sintered ceramic. This increases as the sintering temperature increases, causing further reduction in the ANC.

3.7 Leachate analysis data
3.7.1 Leaching of alkali metals and alkali metal earth ions

Leaching data for Ca, Mg, Na and K (mg/Kg) with variation in leachate pH is shown in Figure 8. Ca dominates the cation component in IBA leachates and these ions leach at high concentrations from milled IBA. The addition of acid releases Ca from CaCO3 as well as other soluble mineral phases that may be present such as CaSO4, CaCl2, CaO and Ca-silicates.

While sintering at 1080 and 1100oC significantly reduces the leaching of these ions at a given leachate pH, sintering at 1020?C produces relatively high levels of Mg and Na under acid conditions. This is in part due to the partial sintering that occurs at this temperature, but also indicates the formation of readily soluble phases containing these ions at 1020?C.

3.7.2 Leaching of heavy metals

Leaching of Cr, Cu, Ni, Zn, Cd, and Pb (mg/Kg) with leachate pH is shown in Figure 9.

Chromium: Chromium shows interesting behaviour, as there is no significant leaching from the IBA sintered at 1080 and 1100oC under very aggressive acid leachate pH conditions. The Cr is very effectively bound into these materials and is not available for leaching. Leaching of Cr from milled IBA and IBA sintered at 1020oC is very different, with leaching rates in the range of 1-10 mg/Kg at leachate pH values below 6. A proportion of the total Cr must be readily available for leaching in the milled IBA. This fraction is not significantly reduced by sintering at 1020oC but is reduced in IBA sintered at 1080 and 1100oC.

Copper: Milled IBA leached Cu at between 1 and 10 mg/Kg over a broad pH range. Sintering at 1020 and 1080oC significantly reduces leaching to very low levels at leachate pH above 3. However Cu does leach from IBA sintered at 1100oC and this material showed significant Cu leaching at relatively high leachate pH. This may be due to decomposition of Cu-containing sulphate salts that is thought to be associated with the sample expansion and formation of isolated porosity observed in samples sintered at this temperature.

Nickel: Leaching of Ni is at low levels over the range of leachate pH values examined. Sintering reduces the leaching compared to milled IBA by approximately an order of magnitude under highly acid conditions.

Zinc: The leaching behaviour of Zn from milled IBA shows the typical behaviour expected, with a significant increase as the pH falls below 6. Sintering generally reduces Zn leaching, although as for Cu there are quite high levels leached from IBA sintered at 1100?C and when IBA is sintered at 1020?C approximately 5 mg/Kg is leached in water (0 eq/Kg of waste).

Cadmium: There was no detectable leaching of Cd from samples sintered at 1080 or 1100oC down to leachate pH values of 3, and this element is therefore effectively not available for leaching from these materials. However the results show that it is leached from IBA sintered at 1020oC and milled IBA in the pH range between 6 and 3 at between 0.1 to 3 mgKg-1.

Lead: Lead leaching generally reduces with sintering although, as for Zn and Cu, there is some evidence of preferential leaching from the IBA sintered at 1100oC.

The proportions of these heavy metals in the IBA leached under aggressive leachate conditions (leachate pH 3) have been derived from the data in Figure 9 and the results summarised in Table 3. Leaching at pH 3 is considered to represent the fraction that is potentially available for leaching, as pH values below this are not normally encountered in the environment (Holbert & Lighty 1998). This clearly demonstrates the reduction in leaching associated with sintering IBA. The most readily available heavy metals are Zn and Pb, and approximately 27 and 20% of the total of these in milled IBA can be considered as potentially leachable under aggressive acid conditions. Sintering under optimum conditions (1080oC) reduces this to below 1%. The percentage reduction in leaching under aggressive conditions resulting from sintering at 1080oC ranged from a minimum of 90% for Ni and 93% for Cu, to greater than 99% for Cr, Cd, Zn and Pb.

4. DISCUSSION

Sintering the less than 8mm fraction of IBA can produce relatively high-density ceramic materials (2.6 g.cm-1) that no longer contain the majority of the original crystalline phases present in IBA. The densification is expected to occur due to liquid phase sintering given the significant level of glassy material present in milled IBA. The effect of sintering is not just to bond together the numerous crystalline and more refractory materials present in the IBA. This type of thermal treatment also induces significant crystalline transformations into the material and causes the formation of a number of common rock forming minerals. Diopside, clinoenstatite and augite are formed in sintered IBA and these belong to the pyroxene group of silicates that occur as stable phases in almost every type of igneous rock and they can be found in rocks of widely different compositions. Diopside and augite are calcium pyroxenes, while clinoenstatite is a magnesium-iron pyroxene. Wollastonite also forms in sintered IBA and is a common mineral in metamorphosed limestones and some alkaline igneous rocks. It often occurs with calcite, tremolite, diopside, anorthite and a number of other rare calcium-magnesium silicate minerals (Deer et al. 1992).

These types of crystalline mineral phases have also been reported to form in glass-ceramics produced by thermally treating mixes of a number of other municipal and industrial wastes including IBA, incinerator fly ash, coal fired power station fly ash, waste fluorescent glass and electric arc furnace dust (Erol et al. 2000, Erol et al. 2001, Öveçolu et al. 1997, Young & Jong 2002, Yeon-Hum et al. 2002, Boccaccini et al. 1995, Boccaccini et al. 1997, Boccaccini et al. 2000, Rincón et al. 1999, Gao & Drummond 1999, Romero et al. 1999).

The effect of sintering is to increase the fraction of heavy metals incorporated in silicate and alumino-silicate minerals compared to the original IBA and as a result they are increasingly unavailable for leaching. There is also expected to be heavy metals incorporated in amorphous phases, but in general these are more readily attacked under acid conditions and so this is likely to contribute to the potentially leachable fraction.

The work also shows that under certain conditions sintering IBA can cause an increase in the leaching of particular metals. This is observed in the leaching behaviour of Cu, Zn and possibly Pb when sintered at 1100oC and Cr leaching from IBA sintered at 1020oC. It is thought that a fraction of the Cu, Zn and Pb may be present in IBA as metal sulphates that decompose just below 1100oC and this may leave the metal in a more leachable form. Cr leaching from incinerator fly ash sintered at 800oC is reported to be highly dependent on the conditions during sintering, as trivalent Cr can be converted to the more mobile hexavalent form during sintering in an oxidising environment (Kuen-Sheng et al. 2001). It is suggested that similar effects may occur to Cr in IBA sintered at 1020oC, while at higher sintering temperatures the Cr remains in the trivalent state and is incorporated into non-leachable phases.

The fraction of the heavy metals present in sintered IBA that leach is relatively low. For example, Zn was present at about 3650 mg/Kg. Approximately 27 weight % of this is potentially available for leaching from milled IBA under severe acid conditions. However, significantly less than 1 weight % of the total Zn can be leached from sintered IBA. Similar reductions occur for the other metals examined, with the smallest reduction in leaching of around 90% occuring for Ni. This work therefore indicates that properly sintered IBA ceramics can be considered as benign materials with respect to leaching of the heavy metals examined under a range of pH conditions.

There is an increasing requirement for lightweight inert pelletised materials that have low water absorption, for use in a range of construction and geo-technical applications. The aim of ongoing research is to engineer controlled levels of closed porosity into sintered IBA, to reduce the density, and allow this material to be fully exploited as a valuable resource.

5. CONCLUSIONS

The fraction of MSW incinerator bottom ash with a particle size less of than 8mm has been processed using conventional ceramic processing involving wet ball milling, drying, powder pressing and sintering to form new ceramic materials.

Samples prepared from milled IBA increased in density with sintering temperature up to 1080oC, and the maximum density obtained was 2.6g/cm3. Firing above 1080oC resulted in a density decrease and sample expansion, associated with the formation of isolated spherical pores.

Quartz and calcite were the major crystalline phases present in milled IBA. Minerals belonging to the pyroxene group were identified as the principal crystalline components in sintered IBA materials. Diopside (CaMgSi2O6) was the major crystalline phase present in high-density sintered IBA, together with significant amounts of clinoenstatite (Mg2Si2O6) and wollastonite (CaSiO3).

The ANC of sintered IBA is reduced compared to milled IBA due to decomposition of calcite (CaCO3) during sintering and incorporation of Ca into both glassy and crystalline phases present in the sintered ceramic.

Sintering can result in a significant reduction in leaching of heavy metal (Zn, Pb, Cu, Cr, Ni, Cd) under acid conditions. Zn and Pb were the most concentrated heavy metals of environmental concern in IBA that leach under aggressive acid conditions, and about 20-30% of the total of these present in the IBA is potentially leachable. Sintering under optimum conditions at 1080?C significantly reduces metal leaching for all metals investigated and typically reduces the leachable fraction to less than 1% of the total of each metal in the IBA.

ACKNOWLEDGEMENTS

This research was funded by the UK Engineering and Physical Sciences Research Council (EPSRC) under the Waste Minimisation through Recycling, Reuse and Recovery in Industry programme (project GR/M 51444).

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Authors
S.Bethanis, C.R.Cheeseman, C.J.Sollars

Centre for Environmental Control and Waste Management,
Department of Civil and Environmental Engineering,
Imperial College of Science, Technology and Medicine,
London SW7 2BU, UK



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