Properties of ceramics prepared using dry discharged waste to energy bottom ash dust

Athanasios Bourtsalas a, c, Luc Vandeperre b, Sue Grimes a, Nicolas Themelis c,

Ralf Koralewska d, Chris Cheeseman a, *

a Department of Civil and Environmental Engineering, b Centre for Advanced Structural Ceramics and Department of Materials,

Imperial College London, London, SW7 2AZ, UKc Earth Engineering Centre, Columbia University, New York, NY 10027, USA

d Martin GmbH für Umwelt- und Energietechnik, Munich, Germany

*Corresponding author: e-mail: c.cheeseman@imperial.ac.uk, Tel.: +44 (0)20 7594 5971

Abstract

The fine dust of incinerator bottom ash (IBA) generated from dry discharge systems can be transformed

into an inert material suitable for the production of hard, dense ceramics. Processing involves the addition

of glass, ball milling and calcining to remove volatile components from the IBA. This transforms the major

crystalline phases present in fine IBA dust from quartz (SiO2), calcite (CaCO3), gehlenite (Ca2Al2SiO7) and

hematite (Fe2O3), to the pyroxene group minerals diopside (CaMgSi2O6), clinoenstatite (MgSi2O6),

wollastonite (CaSiO3) together with some albite (NaAlSi3O8) and andradite (Ca3Fe2Si3O12). Processed

powders show minimal leaching and can be pressed and sintered to form dense (>2.5 g/cm3), hard

ceramics that exhibit low firing shrinkage (<7%) and zero water absorption. The research demonstrates

the potential to beneficially up-cycle the fine IBA dust from dry discharge technology into a raw material

suitable for the production of ceramic tiles that have potential for use in a range of industrial applications.

Keywords: A. Incinerator bottom ash; B. Energy-from-Waste; C. Resource efficiency; D. Pyroxene; E.

Sintering; F. Ceramics; G. Up-cycling

1. Introduction

The combustion of residual waste in Energy-from-Waste (EfW) facilities is widely used in many

developed countries. There are currently 30 EfW facilities operating in Switzerland and ~450 plants in

Europe. EfW processes residual municipal solid waste (MSW), which is the material remaining after the

maximum amount of commercially viable recyclable materials have been extracted, and the heat

generated is used to produce electricity and/or steam and/or hot water for district heating schemes (Nixon

et al., 2013). The incinerator bottom ash (IBA) produced is a heterogeneous mix of ceramic, glass, brick,

concrete, clinker and particles of fused materials. IBA also contains significant levels of ferrous (~ 10 wt.

%) and non-ferrous metals (~1-3 wt. %), mainly Cu and Al.

1

The amount of IBA produced is typically 20 to 25 wt. % of the input residual MSW to an EfW plant,

and it is normally quenched into water on exiting the combustion chamber, producing wet-discharged IBA

(Crillesen et al., 2006; Sabbas et al., 2003). Alternative dry discharge systems operate at some EfW

facilities and these have significant advantages including more efficient metal recovery and lower

operating energy requirements (Allegrinia et al., 2014). A schematic diagram of the Martin grate-based

combustion and dry bottom ash discharge system operating at the Monthey EfW plant in Switzerland is

shown in Figure 1 (Blatter et.al., 2014). A nearly metal-free fine fraction of IBA (<1 mm) is separated by

an air separator and a dust extraction. The coarse fraction, in which almost all metals are present,

undergoes appropriate separating processes in order to extract the metals.The fine IBA dust from the

Monthey plant is currently reused as a cement substitute together with cement for stabilizing neutral

washed filter ashes prior to disposal in a hazardous landfill (Koralewska et.al., 2014).

The production of ceramics and glass-ceramics from IBA has been extensively reported. The ceramic

manufacturing process normally involves milling raw materials to reduce particle size and increase

sintering reactivity, spray drying to produce granules suitable for pressing, and sintering of the compacted

powder. The production of glass-ceramic involves the production of a glass by melting the raw materials

at high temperature, followed by subsequent heat treatment at lower temperatures to induce

crystallization effects. A major advantage of these types of processing is their ability to encapsulate

heavy metals in the ceramic or glass-ceramic matrix and this results in low metal leaching (Rawlings et

al., 2006).

Glass-ceramic production from IBA has previously been reported (Appendino et al., 2004; Ferraris et

al., 2001). IBA has been mixed with other waste materials to produce glass-ceramics (Aloisi et al., 2006)

and vitrified IBA has been used as a substitute for feldspar in porcelain stoneware production (Rambaldi

et al., 2010). Sintered ceramics have been produced by mixing IBA with clay (Schabbach et al., 2012),

marine dredging spoil (Baruzzo et al., 2006) and water treatment sludge cake (Lin et al., 2006). Sintered

ceramics have also been produced using the <8 mm fraction (Bethanis et al., 2004; Cheeseman et al.,

2003; Bethanis et al., 2002) and the <4 mm fraction of wet discharge IBA (Bourtsalas et al., 2015) . The

research reported in this paper has focussed specifically on processing the fine IBA dust produced from

the Martin dry discharge system and aimed to produce an inert powder suitable for the production of

ceramic tiles.

2. Experimental procedure

2.1. Characterisation of fine IBA dust

A representative 50 kg batch of the <1 mm fine IBA dust was obtained from the Monthey EfW facility

in Switzerland. This represents about 10 wt. % of the total IBA produced from the plant. X-ray

fluorescence (XRF, Spectro 2000 Analyser) was used to characterise the chemical composition. Sub-

samples were dried at 105 ºC and disc milled (Gy–Ro, Glen Creston Ltd., UK) to form powders with a

mean particle size of 35 µm that were pressed into discs. X-ray diffraction (XRD) was used to

characterise the crystalline phases in milled powders (Philips PW 1830 diffractometer with PW1820

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goniometer using Cu Kα radiation with an accelerating voltage of 40 kV). Thermogravimetric analysis

(TG/DTA, Stanton Redcroft, STA-1500 Series) used 25 µg samples of as-received fine IBA powder

heated at a ramp rate of 10 °C.min-1.

2.2. Ceramic processing of as-received powders

The production of ceramic samples involved wet milling the fine IBA dust with different additions of

waste soda lime silica glass for 24 hours in a porcelain ball mill using high-density alumina milling media,

a water to charge ratio of 2 and a milling media to charge ratio of 5. 1 wt. % of polyethylene glycol (PEG-

8000) was added as a binder to aid pressing. Milled slurries were dried overnight at 105 °C and passed

through a 500 µm sieve to form a powder suitable for pressing. Tile samples (110 m × 55 mm × 20 mm)

and disc samples (40 mm diameter) were produced by uniaxial pressing at 48 MPa (Nannetti S hydraulic

press) in steel dies. Pressed samples were then sintered in an electric furnace (Lenton Thermal Design

Ltd., ECF 12/45) at a heating rate of 6 °C .min-1 to temperatures between 1020 and 1100 °C with a dwell

time of 1 hour at peak temperature.

2.3. Production of calcined milled powders

Initial trials using as-received IBA powder produced compacted samples that exhibited high

shrinkage during firing. This is not ideal as high shrinkage tends to result in product warping and high

levels of fired scrap. Therefore the effect of calcining the powder after wet ball milling, drying and sieving

was investigated. This involved heating milled IBA: glass powders in an alumina crucible to temperatures

between 600 and 1200 °C. The calcined powders were then lightly ground using a mortar and pestle and

formed into ceramics using the same processing and firing procedures as for the as-received powders.

2.4. Physical property characterisation of products

The linear shrinkage (LS) occuring during sintering was determined from disc samples using:

LS = (Do – Ds)*100/ Do

where Do is the diameter of the green (unfired) disc and Ds is the diameter of the sintered disc.

The density of the sintered samples was determined by Archimedes method, taking into account the

effect of temperature on the specific gravity of water. The water absorption of sintered samples was

determined from the increase in weight of dried samples after immersion in water for 24 hours under

vacuum to remove the air entrapped within the open porosity (B.S 812: Part 2, 1995).

The mechanical properties of the optimum sintered ceramic were further characterised and these

were compared to the commercial terracotta ceramic tiles.

Vickers hardness (HV) was measured (Zwick/RoellIndentec ZHV instrument, Germany) using a

500 g load with a penetration time set to 10 s, with the average taken from seven measurements.

3

Thermal conductivity of the optimum ceramic samples was determined using a TT–TC Probe (Therm

Test Inc.). This is a non-destructive test based on the Mathis modified hot-wire technique that measures

the temperature rise at a defined distance from a linear heat source in contact with the test material. The

heat source is assumed to have a constant and uniform output along the length of the test sample and

the thermal conductivity is then derived directly from the resulting change in temperature over a known

time interval (Ferrandiz-Mas et al., 2014) .

Microstructural analysis used optical microscopy (LOM, Olympus, BX 51TRF) of polished samples.

X-ray diffraction used the same procedures as for the as-received fine IBA dust to determine the

crystalline phases present in milled, calcined and sintered samples.

2.5. Metal leaching from as-received fine IBA dust and sintered ceramics

The leaching characteristics of the as-received fine IBA dust, and the optimum sintered ceramic

have been evaluated using the pH dependence leaching test using a liquid to solid (L/S) ratio of 10 (CEN/

TS 14429). The CEN standard allows characterisation of leaching under different pH conditions covering

the range from basic to acidic. Samples for leaching were dried and ground to pass through a 150 μm

sieve and 2 g of solid samples were mixed with 20 ml of aqueous acid solutions that varied in

concentration between 0 and 100% 1.0 N nitric acid over 15 equal increments. The slurries obtained

were mixed for 48 hours in a rotary extractor. The leachate was extracted and filtered through a 0.45 μm

membrane filter and analysed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES)

for a range of elements including Ca, Na, Mg, K, Zn, Pb, Cu, and Al.

3. Results

3.1. Characterisation of the as-received fine IBA dust

Chemical analysis and loss on ignition data is given in Table 1. Fine dry discharged IBA dust

contains significant Si, Fe, Ca and Al. XRD data given in Figure 2 shows that the major crystalline phases

present were quartz (SiO2), calcite (CaCO3), gehlenite (Ca2Al2SiO7) and hematite (Fe2O3).

Thermogravimetric analysis data in Figure 3 shows that from ambient temperature to 120 °C weight

loss is due to evaporation of residual moisture. Significant weight loss occurs between 550 and 850 °C

due to decomposition of calcite (CaCO3) to CaO and CO2 and other volatile phases (Slavo et al., 2000).

The decomposition of trace levels of alkali metal sulphates with the evolution of SO 2 is also reported to

occur at temperatures between 950 and 1000 °C (Chimenos et al., 1999; Mangialardi et al., 1998; Stern

and Weise, 1966).

3.2. Physical properties of IBA ceramics

The effect of sintering temperature on the density, linear shrinkage and water absorption of IBA:

glass samples produced from uncalcined powders is shown in Figure 4. For the uncalcined samples

4

maximum densities between 2.20 and 2.40 g/cm3 were obtained when sintering between 1020 and 1040

°C for a mix containing 30 wt. % glass. These samples exhibited high (~20%) linear shrinkage and this

tended to cause deformation and warping of tile samples. The water absorption of the uncalcined 30 wt.

% glass samples sintered between 1020 and 1040 °C was between 10 and 15%.

The effect of calcining temperature on the density, firing shrinkage and water absorption of samples

sintered at 1080 °C with varying glass additions is shown in Figure5. Significantly higher densities

between 2.30 and 2.80 g/cm3 are achieved by calcining powders above 1000 °C. Calcining powders

below 1000 °C produced ceramic samples with densities between 1.50 to 2.30 g/cm 3. The linear

shrinkage that occurs during sintering decreased with increasing calcining temperature for all mix

compositions. Higher density and lower shrinkage results from reducing the volatiles present in the IBA

by calcining at temperatures above 1000 °C and this also improves the packing of calcined powders

during pressing to give higher pressed densities. The water absorption of samples prepared from

calcined powders containing more than 20 wt. % glass was negligible.

Maximum fired densities between 2.40 and 2.80 g/cm3 were obtained for compositions containing 20

and 30 wt. % of glass calcined at 1080 °C and sintered at temperatures between 1040 and 1080 °C.

Linear shrinkage was significantly reduced (<10%) compared to un-calcined samples for all mix

compositions. The water absorption was negligible for samples containing > 10 wt. % glass sintered

between 1080 and 1100 °C.

Optical micrographs of polished surfaces of sintered ceramics prepared using optimum firing

conditions are shown in Figure 6. All samples contain porosity but ceramics produced with low glass

content contain more and larger pores, consistent with lower density data. Sample compositions

containing 20 and 30 wt.% glass had low porosity and improved appearance compared to samples with

lower glass additions. The densities of samples produced with 20 and 30 wt.% glass were similar at ~2.7

g/cm3 and these samples had a linear shrinkage of < 7%, which is lower than the typical firing shrinkage

of terracotta ceramics.Since the objective of the research is to utilise the fine IBA dust from a dry

discharge system, the mixture composition containing 20wt.% glass was chosen as the optimum and this

was further characterised.

The mechanical properties of optimum ceramics produced from dry-discharged fine IBA dust

(calcined at 1080 °C and sintered at 1080 °C containing 20wt.% glass) compared to terracotta ceramics

are shown in Table 2. This demonstrates the improved properties of the ceramic manufactured from fine

dry-discharged IBA dust in comparison to terracota ceramics (Acchar et al., 2006).

X-ray diffraction data for 24 hour milled 80wt. % IBA, 20wt. % glass and the corresponding sintered

ceramic fired at 1080 °C is shown in Figure 7. Quartz (SiO2), calcite (CaCO3), hematite (Fe2O3) and

ghelenite (Ca2Al2SiO7) are the major crystalline phases in the as-received fine IBA dust, and these were

transformed into the pyroxene minerals diopside (CaMgSi2O6) clinoenstatite (MgSi2O6) and wollastonite

(CaSiO3) together with some albite (NaAlSi3O8) and andradite (Ca3Fe2Si3O12) in the sintered product.

3.4 Effect of processing on the leaching characteristics of fine IBA dust and sintered ceramic

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The effect of processing dry-discharged fine IBA dust on the acid neutralisation capacity (ANC),

expressed as final leachate pH as a function of acid addition in equivalents of acid per kg of solid is

shown in Figure 8. A significant reduction in ANC was exhibited for the ceramic sintered at 1080 °C and

produced with 20 wt. % glass powder calcined at 1080 °C compared to the as-received IBA. The

concentrations of selected alkali and alkali earth metals and heavy metals leached from both the as-

received fine IBA dust and sintered at 1080 °C ceramic produced with IBA: glass powder calcined at

1080 °C are shown in Figure 9 and 10. Up-cycling of dry-discharged fine IBA dust significantly reduced

leaching of all elements contained in the as-received fine IBA dust.

4. Discussion

There are currently limited options for the reuse and up-cycling of the fine fraction of dry-discharged

IBA dust, using ceramic processing involving milling fine IBA dust blended with waste glass, calcining,

pressing and sintering, makes it possible to produce ceramics with advanced properties suitable for use

in a range of potential applications.

The calcining stage involves heating the milled IBA and glass to above 1000 ºC and this transforms

the crystalline phases present in the IBA so that post calcining the powder formed is no longer the

original fine IBA dust. XRD data indicate that the original fine IBA dust contains quartz (SiO 2), calcite

(CaCO3) gehlenite (Ca2Al2SiO7) and hematite (Fe2O3) as the major crystalline phases. Post calcining the

crystalline phases present are predominantly the pyroxene-group minerals diopside (CaMgSi2O6),

clinoenstatite (MgSiO3) and wollastonite (CaSiO3) together with some albite (NaAlSi3O8) and andradite

(Ca3Fe2Si3O12). The heating the milled fine IBA dust glass mix also volatilises trace phases that would

otherwise cause problems during sintering. As a result a much denser ceramic microstructure is formed

from sintering the calcined powder in comparison to the as milled powder as can clearly be seen in

Figure 5.

Ceramics produced using the optimum processing conditions also exhibit significantly reduced linear

shrinkage during firing and this is associated with a higher ‘green’ density in the pressed but unfired

compacted sample. High shrinkage during firing of ceramics tends to be associated with warping and

deformation of sintered products. The percentage shrinkage of the optimum ceramics were similar or

slightly lower than the shrinkage reported for typical terracotta ceramics. Strong interdependencies

between the linear shrinkage, the density and the water absorption were observed, and this is in

accordance with the observations of other researchers (Velis et al., 2014; Aloisi et al., 2006; Cheeseman

et al., 2003; Barbieri et al., 2002). Shrinkage during sintering and the associated increase in density is

driven by the elimination of surface area in the powder compact and this is also significantly aided by the

fluxing effect of the added waste glass which promotes liquid phase sintering.

The pyroxene group of minerals found in the processed IBA glass mix are of two types, the calcium

pyroxenes, which include diopside and wollastonite and the magnesium pyroxene, which includes

clinoenstatite. Pyroxenes are an important group of rock-forming silicate minerals of variable composition

typically containing Ca, Mg and Fe. They occur as stable phases in almost every type of igneous rock

and are found in rocks of widely different compositions. Several studies have reported the formation of

6

diopside as the major crystalline phase formed from vitrified municipal incinerator fly ash, filter dusts, IBA

and thermal power plant fly ash. Wollasonite and clinoenstatite have also been reported to form in other

waste derived ceramics (Rawlings et al., 2006; Romero et al., 2001).

The as-received dry-discharged fine IBA dust exhibited a fairly constant rate of pH decrease with

acid addition. Carbonation during ageing of the fine IBA dust, leads to the formation of CaCO 3, which

provides significant buffering capacity (Van der Sloot et al, 2012; Guimaraes et al, 2006). The sintered

ceramics show rapid decline in leachate pH with acid addition, indicating reduced ANC compared to the

as-received fine IBA dust. Calcining decomposes CaCO3 to CaO and CO2 and the CaO then forms new

amorphous and crystalline phases in the processed fine IBA dust :glass mix (Van der Sloot et al, 2012;

Guimaraes et al, 2006).

Leaching of Ca occurs from soluble minerals phases present in the as-received fine IBA dust, such

as CaCO3, CaSO4, CaCl2, CaO and Ca-silicates. The ceramics produced using optimum processing

leach much less Ca due to the formation of low solubility calcium silicates, such as diopside, wollastonite

or andradite. Na, K and Mg leach from the as-received fine IBA dust over a wide pH range and this is

also significantly reduced from optimum sintered ceramics. The similarity in the leaching behaviours of

Mg and Ca from the as-received fine IBA dust, is in accordance with the role of calcite in controlling Mg

leaching (Van der Sloot et al, 2012).

The optimum processed ceramics exhibit low leaching down to very aggressive acid leachate pH

conditions. Cu, Zn and Pb are incorporated within the phases formed during sintering. Cu leaching was

reduced to very low levels after processing the as-received fine IBA dust and glass. Zn is present in the

fine IBA dust:glass mix as zinc silicates and aluminates, which are highly soluble. Pb is also readily

leached from the as-received fine IBA dust. The percentage reduction in leaching resulting from

processing fine IBA dust and glass under optimum conditions was greater than 99% for all the metals

examined.

5. Conclusions

The fine fraction of dry-discharged IBA dust can be processed to produce pyroxene group ceramics.

Processing steps involve mixing IBA with glass which is added to aid liquid phase sintering, wet ball

milling to reduce the particle size distribution and homogenise the mix and calcining to transform the

crystalline phases present and enable volatilisation of gases responsible for pore-forming. Calcining

significantly improves the pressing and sintering behaviour of fine IBA dust. High linear shrinkage was

observed when sintering as-received IBA powders and this results in deformation and warping of the

ceramics tiles formed. Optimum sintered ceramics exhibit low firing shrinkage (< 7%), high densities (>

2.7 g/cm3), negligible water absorption and improved mechanical properties. Quartz and calcite are the

major crystalline phases present in the as-received fine IBA dust. The pyroxene group minerals, diopside

(CaMgSi2O6), clinoenstatite (MgSiO3) and wollastonite (CaSiO3) are the major crystalline phases present

in calcined powders and ceramics sintered at 1080 °C. Calcining and sintering at optimum processing

temperatures significantly reduces the ANC compared to as-received fine IBA dust. Optimum sintered

ceramics from dry-discharged fine IBA dust show minimal leaching compared to the as-received IBA.

7

Acknowledgements

This research was funded by the Global Waste to Energy Research and Technology Council

(WTERT), Wheelabrator Technologies Inc. and Martin GmbH für Umwelt- und Energietechnik. AB was

partly funded through an Imperial College Dixon Scholarship. We also wish to acknowledge the support

of Adam Day of Day Group and Dave Parks from Monier Group.

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Table 1.

Chemical composition determined by XRF of the as-received fine IBA dust from dry discharge

technology. Others refer to minor and trace elements present in the as-received fine IBA dust, such as

Ba, Cd, Co, Cr, Mn, Ni, Pb, Sr and Zn.

Oxide Weight percent% (dry basis)

Oxide Weight percent% (dry basis)

SiO2 36.3 Na2O 2.0Fe2O3 18.4 P2O5 1.9CaO 15.7 ZnO 1.9Al2O3 6.4 CuO 1.8SO3 2.2 TiO2 1.8K2O 3.7 ClO2 1.4MgO 3.4 Others 2.3

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Table 2. Properties of optimum ceramics (80wt.% IBA: 20 wt.% glass) calcined and sintered at

optimum temperature (1080 °C) compared to terracotta ceramics.

Mechanical property Sintered ceramic

Terracotta ceramics

Density (g/cm3) 2.7 2.3

Young’s modulus (GPa) 76.2 71.0

Vickers microhardness (GPa) 4.4 3.1

Thermal conductivity (W/m*K) 0.6 0.6

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Fig. 1. Cross section of a grate-based Martin combustion and dry bottom ash discharge system.

12

Fig. 2. X-ray diffraction traces for the as-received fine IBA dust from dry discharge system. Schematic

X-ray powder diagrams are included for relevant phases. The data are all plotted with the same intensity

scales.

13

Fig. 3. Thermogravimetric curve for the as-received fine IBA dust from dry discharge technology

heated to 1000 °C with a 10 °C.min-1 ramp rate.

14

(a)

(b)

(c)

Fig. 4. Effect of sintering temperature (1 hour dwell) on the: (a) density, (b) linear shrinkage and (c)

water absorption of ceramics produced with uncalcined IBA: glass powders.

15

(a) (d)

(b) (e)

(c) (f)

Fig. 5. Effect of calcining (a-c) and sintering (d-f) temperature (1 hour dwell) on the density, linear

shrinkage and water absorption of ceramics sintered at 1080 °C and ceramics produced with IBA:glass

powders calcined at 1080 °C, respectively. The lines in the graphs represent the properties of ceramics

produced with uncalcined 80wt. % IBA:20wt. % glass powder sintered at 1080 °C.

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(a)

500μm (b)

500μm

(c) 500μm (d)

500μm

Fig. 6. Optical micrographs of polished surfaces of ceramics calcined and sintered at optimum

temperature (1080 °) containing (a) 70wt. %, (b) 80wt. %, (c) 90wt. % and (d) 100wt. % fine IBA

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Fig. 7. X-ray diffraction traces of optimum ceramics (80wt.% IBA: 20 wt.% glass) calcined and sintered

at optimum temperature (1080 °C). Schematic X-ray powder diagrams are included for relevant phases.

The data are all plotted with the same intensity scales.

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Fig. 8. Acid Neutralisation Capacity data for as-received fine IBA and optimum ceramics (80wt.% IBA:

20 wt.% glass) calcined and sintered at optimum temperature (1080 °C).

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(a) (b)

(c) (d)

Fig. 9. Leaching data in mg/kg for (a) Ca, (b) Na, (c) Mg and (d) K as a function of leachate pH of as-

received fine IBA dust from dry discharge system and optimum ceramics (80wt.% IBA: 20 wt.% glass)

calcined and sintered at optimum temperature (1080 °C).

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(a) (b)

(c) (d)

Fig. 10. Leaching data in mg/kg for (a) Cu, (b) Pb, (c) Zn and (d) Al as a function of leachate pH of as-

received fine IBA dust from dry discharge system and optimum ceramics (80wt.% IBA: 20 wt.% glass)

calcined and sintered at optimum temperature (1080 °C).

21