Properties and Microstructure of Sintered Incinerator Bottom Ash

Properties and Microstructure of Sintered Incinerator Bottom Ash

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Properties and Microstructure of Sintered Incinerator Bottom Ash
ABSTRACT

The less than 8mm fraction of bottom ash produced at a commercial MSW waste incinerator has been milled, pressed and sintered at different temperatures to form new ceramic materials. The effects of the milled ash particle size distribution, powder compaction pressure and sintering temperature have been investigated, with the materials formed being characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermal analysis (TG/DTA). The milled ash was found to contain quartz (SiO2), calcite (CaCO3), minor amounts of ghelenite (Ca2Al2SiO7) and hematite (Fe2O3) and had a loss on ignition of 12.1%. Sintered densities of materials produced from ashes milled to 95% less than 27?m increased from 1.38 to 2.63g/cm3 as the sintering temperature increased from 1020oC to 1080oC. Firing above 1080oC caused a rapid decrease in density and sample expansion. This was associated with the formation of a significant volume of approximately spherical closed pores. Diopside (CaMgSi2O6) was the principal crystalline phase in the high-density materials, with only minor amounts of quartz and magnetite (Fe3O4). Incinerator bottom ash can therefore be processed to form ceramics with properties controlled by the ash particle size distribution and sintering conditions.

Introduction

Disposal of waste to landfills is referred to as the lowest priority in the waste management hierarchy, since significant economic and environmental benefits can accrue from less usage of raw materials, maximisation of reuse and recovery of waste, and minimisation of their disposal. About 70% of controlled waste (excluding sewage sludge and dredged spoils) go to landfill. This has been a cost-effective option in the past but, since 1996, landfill taxes, increasing awareness of the long-term liabilities that could be held by the operators have reduced its attractiveness (1-Landfill tax). Although landfill remains the dominant disposal method for municipal solid waste (MSW), many industrialised countries are increasingly turning to waste incineration as a viable alternative. This is because in many parts of the world readily available landfill void space is becoming exhausted and there are often significant problems associated with locating new landfill sites. The potential long-term adverse environmental effects and costs of landfill are also now widely recognised.

Incineration has the advantage that the plant can be located close to where waste is generated and it can use the significant energy content of MSW to produce electricity and/or hot water for distribution to the local community. This can now be efficiently achieved in modern 'energy from waste' (EfW) plants that are designed to have minimal environmental impact (2). MSW incinerators mainly handle domestic waste, operate at furnace temperatures of 800-1100oC and generate large quantities of solid wastes. Disposal of MSW incinerator ashes to landfill occupies only one-tenth the volume of the original waste, conserving void space. However, avoiding landfill disposal of incinerator ash by developing reuse applications is clearly the preferred option and has been the subject of extensive research (3,4,5).

As-produced bottom ash is a highly heterogeneous burnt-out mixture of slag, ferrous and non-ferrous metal, ceramics, glass, other non-combustibles and residual organic matter (6,7,8). It contains small amounts of polychlorinated dibenzo-p-dioxins/polychlorinated dibenzofurnas (PCDDs/PCDFs) (6). The extreme heterogeneity of bottom ash combined with the potential leachability of heavy metals means that some form of processing is likely to be required to improve the characteristics of the ash for use as a replacement for the natural lightweight aggregates used for construction purposes. The development of "value added" re-uses would have particular benefits both in terms of resource recovery and protection of the environment. For example, processing ash to form lightweight aggregate would have definite advantages, as the extraction of natural aggregate causes environmental damage (9,10). With this in mind, the UK government has announced the introduction of a tax on aggregate extraction that may act as further incentive for the development of alternative 'secondary' or 'waste derived', manufactured aggregates.

High temperature sintering can convert particulate inorganic waste materials into more environmentally acceptable solid-monolithic products with improved properties for potential reuse. The effect of sintering air pollution control (APC) residues produced at a MSW incinerator has been investigated and was reported to reduce leaching of Pb, Cd and Cr (11). Lightweight aggregates have been manufactured by pelletising and sintering power station fly ash, sewage sludge ash and industrial sludges (12,13,14).

The aim of this work was to investigate the properties of materials produced by sintering milled incinerator bottom ash. The effects of firing temperature and particle size distribution of the ash on the physical properties, mineralogy and microstructure of uniaxially pressed and sintered samples have been studied, and is part of ongoing research into the manufacture of new materials containing incinerator ashes.

Experiments

2.1 Material and processing

MSW bottom ash produced by the South-East London Combined Heat and Power (SELCHP) EfW plant was used for this research. This is one of the largest plant currently operating in the UK, with a capacity to burn 420,000 tonnes per year of household waste. It uses well-established mass-burn incineration technology and generates 34MW of electricity. The bottom ash had been weathered for between six to eight weeks and the majority of the ferrous and non-ferrous metals removed by magnetic separation. The remaining ash was sieved to separate different size fractions. The sample of ash used in these experiments had a particle size
Ball milling further reduced and controlled the particle size distribution of the bottom ash. This processing technique is widely used in the ceramics industry and has the advantage that in addition to reducing the particle size it produces an homogenous slurry more suitable for subsequent processing. Ash samples were processed using a 3 litre polypropylene mill, rotating at approximately 50rpm, containing high-density alumina milling media. Batches of 500g of ash were wet milled for 2, 8 and 16 hours using a water to solids ratio of 2. The particle-size distribution of milled slurries in the range 0.4 to 900?m were determined using laser diffraction (Beckman Coulter, LS-100).

Milled slurries passing a 355?m sieve were de-watered by pressure filtration on a Whatman GF/F filter paper using a stainless steel extraction vessel. The filter cake formed was oven-dried for 16 hours at 105oC and ground in a pestle and mortar to produce a fine, consistent, grey powder.

2.2 Milled ash characterisation

The chemical composition of the milled powder was determined by mixing with a lithium metaborate and tetraborate flux, heating to 1000oC and dissolving in nitric acid. This was then analysed using inductively coupled plasma atomic emission spectroscopy (ICP-AES). Crystalline phases present in the dried milled ash were analysed by XRD (Phillips PW1710 diffractometer) using 50mA and 40Kv, CuK? radiation. The loss on ignition was obtained by heating dried milled ash to 550oC.

The thermal stability of the milled ash was characterised from 25 to 1250oC using a Stanton Redcroft thermal analyser (STA-1500 Series) at a heating rate of 6oCmin-1 in air, using alumina as reference material. Weight and energy changes in 15mg samples were recorded as thermogravimetric (TG) and differential thermal analysis (DTA) data.

2.3 Production of sintered samples

The milled dried bottom ash powder was uniaxially pressed at 32MPa using a stainless steel die, to form 20mm diameter and approximately 22mm high cylindrical “green” samples. These were sintered at temperatures ranging from 1020 to 1100oC, using a ramp rate of 6oCmin-1 and a dwell time of 1 hour. In addition, the effect of compaction pressure on the physical properties of the unfired ("green") and sintered samples was investigated by pressing 16 hour-milled ash at 4, 8, 16, 32 and 64 MPa prior to sintering at 1080oC for 1 hour.

2.4 Characterisation of sintered specimens

Dry density, shrinkage and water absorption were determined on samples sintered at a range of temperatures. Density was calculated using Archimedes' method and water absorption capacity determined from the increase in weight of 'surface dry' samples after they had been submerged in water for 24 hours. The percentage shrinkage of the sintered specimens was determined from the reduction in sample diameter. Sintered samples were ground to
Results and Discussion

3.1 Particle size distribution data

The effect of milling time on the particle size distribution of the bottom ash is shown in Figure 1 and Table 1, which show data for 2, 8 and 16 hour-milled ash compared to the as-received ash. Fifty percent of the volume (d50 value) of the as-received ash particles was finer than 298?m, and this decreased to a d50 of 15?m after 2 hours milling. Corresponding d50 values for the 8 and 16 hour milled ash were 6.2 and 3.9?m, with 95% less than (d95 values) of 26.9 and 19.5?m respectively.

3.2 Chemical composition of milled ash

Data from repeated chemical analysis of milled bottom ash powder is summarised in Table 2. All samples contained significant levels of silica, alumina, iron and calcium oxides. Sodium, potassium, magnesium, phosphate and titanium were also present. Heavy metals of particular environmental concern that had high concentrations included Zn (>2000 mg/Kg), Ni (80-120 mg/Kg), Cu (650-800mg/Kg) and Cr (300-350mg/Kg).

The pH of the milled ash slurries increased with milling time from 9.4-9.9. This solution did not contain significant levels of Cl but S concentrations increased from 2.2mg/L after 1 hour to 18.6mg/L after 24 hours milling. The loss on ignition of dried milled ash samples ranged from 11.8 to 12.3 weight percent and indicates that a significant fraction of unburnt organic carbon remained in the ash.

3.3 Thermal behaviour of milled powder

TG/DTA data of milled ash is shown in Figure 2. Initial weight loss from ambient to 120oC is due to evaporation of remaining moisture. Volatilisation and oxidation of the organic carbon residue remaining in the ash is likely to occur over a broad range of temperatures up to 1100oC and studies on other organic containing wastes have reported weight losses to 550oC resulting from the release of occluded gases, primarily CO2 (15). A significant weight loss, accounting for 5.3% of the original sample weight, occurs between 650 and 750oC. Samples heated to 650 and 750oC for 1 hour were analysed by XRD and this confirmed decomposition of calcite (CaCO3) to lime (CaO) with consequent loss of CO2. A further weight loss of 1.3% occurs between approximately 1050 and 1140oC. This may be due to the evolution of gaseous SO2 from the decomposition of alkali metal sulfates (16,17).

3.4 Physical properties of sintered samples

The effect of temperature on the density and shrinkage of sintered milled incinerator bottom ash samples is shown in Figures 3 and 4 and Table 3. The particle size controls both density and shrinkage, as more extensively milled ash samples have higher fired densities and shrinkage. The density increases with sintering temperature up to 1080oC for 8 and 16 hour-milled samples. Maximum densities of 2.6g/cm3 were achieved for samples exhibiting 20% shrinkage. Above 1080oC there was a rapid decrease in density and an associated expansion of samples. The coarser particle size distribution in the 2 hour-milled samples produced much lower sintered densities and the increase in density with sintering temperature was greatly reduced.

Figure 5 shows the effect of sintering temperature on water absorption. This decreases with increasing temperature due to a reduction in the open, connected porosity. The 8 and 16 hour-milled samples become practically impermeable when sintered above 1080oC, whereas the 2 hour-milled samples show relatively high water absorption at all firing temperatures associated with a lower density, porous microstructure.

The effect of compaction pressure on “green” densities, sintered densities and shrinkage of 16 hour-milled ash samples sintered at 1080oC for 1 hour is shown in Figure 6. Increasing the compaction pressure had negligible effect on sintered densities, but shrinkage does depend on “green” density and therefore compaction pressure. Samples compacted at higher pressures had increased “green” densities and showed less shrinkage than samples compacted at low pressures.

3.5 Mineralogy of milled and sintered ash

XRD data of the milled ash and the sintered products are shown in Figure 7 and Table 4. The milled ash contained quartz (SiO2), calcite (CaCO3) with a minor presence of ghelenite (Ca2Al2SiO7) and hematite (Fe2O3) and these minerals have previously been identified as major components present in incinerator bottom ashes (8). The intensity of the quartz and calcite peaks was found to decrease significantly when the ash was sintered above 1020oC. Analysis of the 16 hour-milled ash sintered at 1080oC that had maximum density revealed that diopside (CaMgSi2O6) was the major crystalline phase, with only minor amounts of quartz and magnetite (Fe3O4), together with hematite and albite (NaAlSi3O8). Diopside is an important rock forming mineral in several metamorphic and basic to ultra-basic igneous rocks, and is a part of an important solid solution series of the pyroxene group. It is reported to form as the major crystalline phase in glass-ceramics formed from vitrified municipal incinerator fly ash and bottom ash (18,19). Other crystalline phases present as minor phases in the sintered ash were difficult to identify from the XRD data.

3.6 Microstructural analysis of sintered samples

SEM micrographs of fracture surfaces of 16 hour-milled incinerator ash samples sintered at 1020, 1080 and 1100oC are shown in Figure 8a, b and c respectively. The samples fired at 1020oC appeared to be poorly sintered with a granular fracture surface due to the increased level of pores. The fracture surfaces of the samples sintered at 1080oC show a much denser microstructure that appeared to be well sintered, although encapsulated interstices between the coagulated ash particles were also clearly evident. The fracture surfaces of samples sintered at 1100oC were very different. The reduced densities and sample expansion are associated with the formation of large numbers of isolated, approximately spherical pores, as seen in Figure 8c. This foamed internal structure is believed to be due to the evolution of SO2 from the decomposition of alkaline metal sulfates, together with softening of the glassy material present in the ash. This was supported from chemical analysis data that indicated a reduction in SO4 levels from 0.47 to 0.07% on increasing the sintering temperature from 1020 to 1100?C.

Conclusions

1. The fine fraction (
2. The particle size distribution of milled MSW incinerator bottom ash and sintering temperature are key process variables controlling sintered density, shrinkage, water absorption and microstructural characteristics.

3. Samples prepared from ash milled to a d95
4. Firing above 1080oC resulted in sample expansion and a density decrease, and was associated with the formation of an extensive volume of spherical porosity due to SO4 evolution and melting of the glassy phase.

5. Quartz and calcite were the major crystalline phases present in the milled MSW incinerator bottom ash. Diopside (CaMgSi2O6) was formed during sintering and was the dominant crystalline phase present in high-density MSW incinerator bottom ash derived materials.

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). Principal co-investigator is Dr J. Newman.

References

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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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