Waste Management 27 (2007) 1200–1206

www.elsevier.com/locate/wasman

Accelerated carbonation of municipal solid waste incineration fly ashes

Xiaomin Li a, Marta Fernández Bertos b, Colin D. Hills a,*
, Paula J. Carey a, Stef Simon b

a
Centre for Contaminated Land Remediation, University of Greenwich at Medway, Chatham Maritime, Kent, ME4 4TB, UK
b
Centre for CO2 Technology, University College London, Torrington Place, London WC1E 7JE, UK

Accepted 8 June 2006

Available online 2 October 2006

Abstract

As a result of the EU Landfill Directive, the disposal of municipal solid waste incineration (MSWI) fly ash is restricted to only a few
landfill sites in the UK. Alternative options for the management of fly ash, such as sintering, vitrification or stabilization/solidification,
are either costly or not fully developed. In this paper an accelerated carbonation step is investigated for use with fly ash. The carbonation
reaction involving fly ash was found to be optimum at a water/solid ratio of 0.3 under ambient temperature conditions. The study of ash
mineralogy showed the disappearance of lime/portlandite/calcium chloride hydroxide and the formation of calcite as carbonation pro-
ceeded. The leaching properties of carbonated ash were examined. Release of soluble salts, such as SO4, Cl, was reduced after carbon-
ation, but is still higher than the landfill acceptance limits for hazardous waste. It was also found that carbonation had a significant
influence on lead leachability. The lead release from carbonated ash, with the exception of one of the fly ashes studied, was reduced
by 2–3 orders of magnitude.
 2006 Published by Elsevier Ltd.

1. Introduction other non-combustibles and unburned organics, whilst the

APC residues are fine particulates collected by the air treat-
The increasing amount of municipal waste produced is a ment systems, and are a mixture of fly ash, lime and car-
worldwide environmental problem and incineration is a bon. Usually APC residues and fly ash are interchangeable.
popular management option, particularly where recycling Fly ash is generally classified as hazardous waste accord-
or reuse are not possible (European Union, 1999). Inciner- ing to the European Waste Catalogue (European Union,
ation can be used to recover energy and reduce the mass 2000; SEPA, 2003) because of the high levels of soluble
and volume of waste by 70% and 90%, respectively, and salts and heavy metals such as cadmium, lead and zinc.
the ash produced can be recycled or disposed of to landfill. The presence of lime also gives fly ash a high alkalinity,
Currently, there are 13 incinerators in the UK with a total resulting in an increased potential for leaching, which is
processing capacity of 2.9 million tons per year. During the important when landfill is concerned.
incineration process, the hot gases produced from the The Incineration Directive (EU 2000/76/EC) has
waste burn are used to generate electricity and/or heat. resulted in more stringent controls on gaseous emissions
The flue gas is treated with dry, wet or semi-dry lime and from incineration, so that more hazardous compounds will
activated carbon for the removal of acid gases such as be retained in the solid residues. It has been reported that
NOx, SOx and CO2 to meet emission targets. Two catego- 314,000 tonnes of fly ash were produced in the UK during
ries of solid residues are produced in incinerator: bottom 1996–2000, of which nearly 90% was sent to landfill (Envi-
ash and air pollution control (APC) residues. Bottom ash ronment Agency, 2002). However, the implementation of
is a heterogeneous mixture of slag, metals, ceramics, glass, the EU landfill Directive (EU, 1999/31/EC) has dramati-
cally reduced the availability of landfill space for these haz-
*
Corresponding author. Tel.: +44 208 331 9820; fax: +44 208 331 9805. ardous wastes. Since July 2004, the number of sites
E-mail address: c.d.hills@gre.ac.uk (C.D. Hills). registered to accept hazardous wastes has decreased from

0956-053X/$ - see front matter  2006 Published by Elsevier Ltd.

doi:10.1016/j.wasman.2006.06.011
 X. Li et al. / Waste Management 27 (2007) 1200–1206 1201

over 200 to less than 10. In addition, the UK government is the mature properties of the ash to develop. Accelerated
increasing the standard rate of landfill tax by at least €4.4/ carbonation may accelerate these natural reactions. In this
1000 kg each year from €22/1000 kg in 2005 in order to paper, an accelerated carbonation treatment is proposed
encourage the reuse and recycling of waste. The long-term for the treatment of MSWI fly ash. The carbonation of
rate of the tax is €51/1000 kg (HM Treasury Budget, 2003). fly ash was investigated and optimized, and the changes
Therefore, the disposal of MSWI fly ash is becoming an to mineralogy and leaching behaviour were examined.
increasingly costly management option. The use of carbon dioxide in a treatment step for both
There have been a number of research projects examin- bottom ash and fly ash would have the added environmen-
ing the disposal of MSWI ash (Polettini et al., 2001, 2004; tal benefit of the permanent sequestration of carbon diox-
Sakai and Hiraokab, 2000; Mangialardi, 2001, 2003; Mizu- ide and open up the possibility of trading in carbon credits
tani et al., 2000; Mulder, 1996). The main techniques cur- for the incineration companies.
rently under investigation, or in use, are thermal
treatment, stabilization/solidification (S/S) and washing- 2. Experiments and analysis
immobilization processes. Thermal treatment, such as sin-
tering or vitrification (Mangialardi, 2001; Polettini et al., The ash samples were supplied by the Environment
2004), is costly and not widely used. The sintered residues Agency and three incinerators in the UK. Their properties
are more hazardous than the untreated fly ash and become and the typical components of ash are listed in Tables 1
more difficult to dispose of (Sakai and Hiraokab, 2000). So and 2. The ash was divided into two parts. One part was
far this technology is only used in a few countries, such as subdivided further for analysis. The other was treated by
Japan, Korea and Sweden (Ecke et al., 2000). The S/S pro- carbonation. The analysis of untreated and carbonated
cess employs cement or other chemical agents (such as fur- ash included moisture content, carbonate content, mineral-
nace slag and soluble phosphate) to immobilize the ogy, pH and leaching test.
contaminants (Eighmy et al., 1997). It is one of the most
commonly used processes for the treatment of solid waste, 2.1. Reaction procedure
sludge and contaminated soil (Polettini et al., 2001; Mizu-
tani et al., 2000; Environment Agency, 2004). New mineral Two reactors were used for carbonation. One was a
phases are generated during the process and the leaching stainless steel chamber incorporating a cooling plate to
properties of the waste are changed. S/S, however, may examine the influence of temperature. Water, used as the
increase the mass and volume of the wastes leading to an coolant, counteracted the heat of reaction and controlled
extra cost of transportation and disposal. Alternatively, the temperature. Changes in pressure and temperature
the contaminants in fly ash can be removed by washing were monitored using digital gauges. The second carbon-
(Mangialardi, 2003; Mulder, 1996). Water or acid is used ation reactor was a closed chamber containing 100% CO2
as solvent and most of the hazardous compounds, i.e., sol- at an RH 75% operated at ambient laboratory temperature
uble salts and heavy metals, are dissolved. Further steps are and was used to ascertain the optimum water/solid (w/s)
then followed to treat the leached residues. A major con- ratio for ash carbonation.
cern with this technique is the large amount of wastewater
generated.
Table 1
Alternatively, many researchers have been investigating Ash samples
the natural or accelerated aging of bottom ash, a process
Sample name Source Water content
which could also be applied to fly ash disposal. Bottom
EA FA1-3 Environment Agency, UK 0–2.5% Aged
ash can be used as a secondary construction material after
SE FA SELCHP, Lewisham, London 0.5% Fresh
several weeks of natural weathering in landfill sites (Envi- CL FA Cleveland Incinerator, UK 0.7% Fresh
ronment Agency, 2002). It is known that a series of physi- KI FA Kirklees Incinerator, UK 0.35% Fresh
cal and chemical changes take place during natural
weathering including hydrolysis, hydration, precipitation/
dissolution, oxidation/reduction and carbonation (Meima Table 2
and Comans, 1997, 1999; Sabbas et al., 2003). Polettini Main elements analysis of SELCHP fly ash
et al. (2003) claimed that treatment of bottom ash using Elements Amount (%)
CO2, i.e., so called accelerated aging, can result in CaO 36.268
improved mineralogical, chemical and leaching properties. Fe2O3 1.054
It has been shown that the formation of carbonate was one K2O 2.034
TiO 0.526
of the major changes affecting the leaching behaviour and
MnO 0.036
acid neutralization capacity of the bottom ash (Chimenos Pb 0.303
et al., 2000). Ba 0.032
Therefore, aging and weathering are known to affect the Zn 0.752
mineralogy, chemical and leaching properties, particularly Cu 0.053
Ni 0.007
the immobilization of heavy metals in waste and allow
1202 X. Li et al. / Waste Management 27 (2007) 1200–1206

Before carbonation, the ashes were dried at 105 C to major elements Al, Ca, K, Na and minor elements As,
constant weight and then mixed with water before being Ba, Cd, Cr, Cu, Fe, Pb, Ni, Zn in leachate were analyzed
placed into the reactor. For each set of experiments, con- by ICP-OES (Horizontal). All analyses were carried out
trol samples were prepared in the same manner and then in triplicate.
sealed and stored under ambient laboratory conditions
for the same time period. The extent of the reaction of 3. Results and discussion
the ash with CO2 was assessed by measuring the gain in
weight between the initial dry powder and the dried prod- 3.1. Carbonation conditions
uct, being proportional to the amount of CO2 that had
combined with the sample. The measured value was com- The variables that have influence on the reaction, such
pared to the weight gain experienced by the control sample. as water/solid ratio, reaction temperature, reaction time,
To examine the influence of reaction temperature on the etc., were studied in this project. Previous work about the
carbonation, the ashes were mixed with water and then influence of reaction time and particle size has been pub-
placed in the reactor in a uniform layer of 3 mm. The reac- lished (Fernandez Bertos et al., 2004b).
tor was closed tightly and filled up to a pressure of 3 bar The carbonation mechanism can be considered a
with a 1:1 mixture of dry CO2 and N2. The reaction tem- sequential reaction expressed by the following equations
perature was controlled by a cooling system. The pressure (Freyssinet et al., 2002):
of CO2 in the reactor was measured and used to calculate
H2 O þ CO2 ! H2 CO3
the consumption of CO2 gas (in moles) by the ash.
To investigate the influence of water to solid ratio (w/s), H2 CO3 ! Hþ þ HCO þ 2
3 ! 2H þ CO3
values from 0.1 to 0.8 on carbonation were studied. Sam- Ca2þ þ CO2
3 ¼ CaCO3
ples of fly ash (5 g) were carbonated for 3 h by exposing
the ash to a 100% CO2 atmosphere at 75% relative humid- It is known that water is necessary to promote the reaction
ity and at a pressure of 3 bar. A saturated solution of NaCl of CO2, but too much water limits the reaction due to the
was used to maintain a constant relative humidity in the blockage of the pores in the solid (Fernandez Bertos et al.,
chamber. The weight gain of the dry SELCHP ash was 2004a). Hydration and dissolution of CO2 occur in the
measured in triplicate. presence of water, as well as the dissolution of Ca2+ ions
Following these initial trials, the ash was carbonated in from the solid phase, which reacts with the CO2 to form
the closed chamber for 3 days at 3 bar pressure of CO2 and calcium carbonate. At low water–solid ratios, the gas per-
a relative humidity of 75%. The samples were then dried in meability is high and the CO2 effectively diffuses into the
order to examine the carbonate content, mineralogy and material. However, with the increase in water content,
leaching properties. the pores in the ash are effectively sealed off. The diffusion
of gas into the pore system is hindered, inhibiting the
2.2. Analytical methods reaction.
The results of reaction with different w/s ratios are
The moisture content was measured after heating in an shown in Fig. 1. The optimum w/s ratio is 0.3 by weight
oven at 105 C (BS 1377-2, 1990). The carbonate content of ash. However, some authors found the optimum ratio
was calculated from the weight loss on ignition between for the carbonation of cementitious systems to be between
450 C and 900 C. Thermogravimmetric and differential w/s 0.06 and 0.20 (Asavapisit et al., 1997; Klemm and
thermal analysis (TG/DTA) of carbonated ash was per-
formed with a Stanton Redcroft STA-780 thermo analyzer
in a temperature range of 20–1100 C at a heating rate of
10 C/min. Mineral phases in ashes were determined by 14
X-ray powder diffraction. The pH was examined according
12 Fly Ash
to BS 1377 part 3:1990.
The compliance leaching test was carried out in accor- 10
weight gain (%)

dance to British Standard BS EN12457: 2002, which is

designed to examine the short-term and long-term leaching 8
behaviour for landfills. It is two-step leaching test with 6
liquid/solid = 10 L/kg. The ash was leached at L/S = 2 L/
kg for 6 h of end-over-end mixing and then filtered. The 4
residues were leached further at L/S = 8 L/kg for 18 h. 2
The eluate was filtered with a 0.45 lm filter paper and then
divided into two solutions. One sample was measured by 0
Ion-Chromatography (DIONEX) for the chloride and sul- 0 0.2 0.4 0.6 0.8 1
w/s ratio
fate content. The other was acidified with nitric acid to
pH < 2 for elemental analysis. The concentration of the Fig. 1. Carbonation as a function of w/s ratio.
 X. Li et al. / Waste Management 27 (2007) 1200–1206 1203

Berger, 1972) and others have successfully carbonated with between the weight gain recorded and the carbonate as
values up to 0.35 (Yousuf et al., 1993). determined by the TG/DTA. The gain in weight is greater
The conversion at different temperatures is shown in than can be explained by the observed increase in carbon-
Fig. 2, which shows the evolution of the reaction with time. ate content, suggesting that other reaction products are
As expected, the reaction proceeded more quickly at the being generated. Fig. 4 shows the CO2 uptake and the car-
beginning and then slowed down. The initial speed of reac- bonate content increase of ashes. The individual fly ashes
tion is higher at higher temperatures, although the final react with CO2, by different amounts within the range of
conversion achieved is lower. The opposite is the case at 2–7% by weight.
lower temperature. The highest weight gain is observed at
21 C. Consequently, the following reactions were carried 3.3. Mineral phases in ash
out at ambient temperature condition.
X-ray diffractograms of the ashes are shown in Fig. 5
3.2. CO2 uptake and carbonate content and were used to determine the mineralogical changes that
took place during carbonation. Table 3 lists the main min-
As in the carbonation mechanism mentioned above, car- eral phases found in ash.
bonate salts were formed during reaction. The DTA curve, The most obvious differences between the diffractograms
shown in Fig. 3, indicates that the loss of pore water in car- of original and carbonated samples are the increase in cal-
bonated ash occurred under 200 C, and the decomposition cite peak intensity and the reduction of the lime/portlan-
of calcium carbonate happens between 450 and 850 C. So dite/Ca(OH)Cl peaks. Untreated ash contains lime/
the weight loss between 450 and 900 C was considered to portlandite and calcium chloride hydroxide (Ca(OH)Cl),
be the carbonate content of the sample. The combustion of as the result of lime addition during gas treatment. These
organics happens between 300 and 450 C, as is character- phases disappear after carbonation. There were also many
ised by a positive DT peak (Johnson et al., 2003). Si–Al–Ca salts identified in the carbonated ashes, including
As previously explained, the extent of carbonation was gehlenite, braunite and larnite. For Kirklees fly ash,
also assessed by measuring the gain in dry weight before hydrated products, such as gehlenite, were also found after
and after the reaction. However, there is a difference carbonation. So the difference of weight gain and carbon-

0.12
21C 12
10
0.09
42C 8C
8
conversion

weight %

62C
0.06 6

75.5C 4
0.03
2
91C
0
0
EA FA1 EA FA2 EA FA3 SE FA CL FA KI FA
0 50 100 150
t (min) weight gain carbonate content increase

Fig. 2. Evolution of reaction at different temperatures. Fig. 4. Weight gain during reaction for the various fly ashes.

Q Quartz G Gehlenite
C
C l Ca(OH)Cl H Halite
100 2 C Calcite Sy Sylvite
weight V Vaterite Ca Lime(CaO)
95 A Anhydrate P Portlandite
delta T 1
90
I in (Counts)
weight (%)

Cl Cl
H
T

85 0 S
carbonated CL FA V G C C C H C G
C V
80
-1 Cl Q Cl Sy Cl
75 original CL FA Cl
P Ca Cl Cl
H
70 -2
0 200 400 600 800 1000 5 10 20 30 40 50 60
temperature (˚C) 2-Theta - Scale

Fig. 3. TG/DTA curve of carbonated SELCHP fly ash. Fig. 5. XRD diffractograms of original and carbonated fly ash.
1204 X. Li et al. / Waste Management 27 (2007) 1200–1206

Table 3 in the pH of the Cleveland ash. Therefore, accelerated car-

Main mineral phases in ash bonation has a major influence on ash pH.
Mineral phases Formula Untreated Carbonated
ash ash 3.5. Leaching properties
p
Lime CaO
p
Portlandite Ca(OH)2 One of the most important criteria for disposal to land-
p
Calcium chloride Ca(OH)Cl
hydroxide
fill or reuse of wastes is the release of hazardous com-
p p pounds to the surrounding environment. The Landfill
Quartz SiO2
p p
Anhydrite CaSO4 (England and Wales) Regulation, 2004 (Statutory Instru-
Halite NaCl
p p ment, 2004) gives criteria for waste landfills, in which the
Sylvite KCl wastes sent to landfill are classified into three categories,
p p
Friedels salt Ca2Al(OH)6Cl(H2O)2
Nordstrandite Al(OH)3
p ‘non-hazardous’, ‘hazardous’ and ‘inert’ in terms of their
p leachability and stability. The limit values for landfill are
Calcite/vaterite CaCO3
Al2O3 Æ 3CaO Æ 2SiO2 given in Table 4. To determine which elements are of con-
p p
Gehlenite Ca2Al2SiO7 cern, the leachability of metals, such as As, Ba, Cd, Cr, Cu,
Ca2Al(Al Æ 5B Æ 5Si Æ 5Cr Æ O7) Mo, Ni, Pb, Zn, and soluble salts Cl, SO4 from untreated
and carbonated SELCHP ash were analyzed. The release
ate content above could be explained by hydration occur- of Cl, SO4, Cd, Cr, Cu, Pb and Zn from the original ash
ring at the same time as carbonation. It should be noted was over the landfill limit values and should be considered.
that there are many chloride bearing phases, such as halite,
sylvite and Ca(OH)Cl, in ash. These phases are the main 3.5.1. Soluble salts in leachate
components releasing chloride in leachates (Bodenan and Sulfate and chloride are the main soluble salts released
Deniard, 2003) and their high solubility leads to high con- from ash. The existence of large amounts of these salts in
centrations of chloride in ash leachates. fly ash has a major influence on the setting and hydration
of cement (Taylor, 1990).
3.4. PH of ashes From the mineralogical analysis of ash, the chloride ions
occur in halite and sylvite, both of which are highly soluble.
Due to the existence of lime, MSWI fly ash is highly As the result, chloride release from fly ash is much higher
alkaline, which is detrimental to its reuse. After carbon- than the acceptance value for hazardous wastes (Table 4)
ation, however, the alkalinity of ash is expected to be as shown in Fig. 7. Although carbonation reduced the chlo-
reduced as the calcium oxide is transformed to calcium car- ride mobility, it was still higher than the acceptance value.
bonate. Fig. 6 shows the pH change after carbonation. Sulfate content in the leachate is under the limit for non-
The original pH of ash is around 12–12.5, which is very hazardous wastes for all of the ash samples. The sulfate
close to the pH of a solution saturated in portlandite. The concentration was lower after carbonation as shown in
alkalinity of aged ashes from the Environment Agency is Fig. 8. There was no obvious correlation between ash car-
slightly lower than fresh ash, which may be the result of bonation reactivity and leachable sulfate.
natural aging. After carbonation, the pH of these aged
ashes was lowered to 7–10. The pH drop of fresh ash is, 3.5.2. Heavy metals release
to a large extent, related to their carbonation reactivity, The concentrations of Ca, Na and K in leachates are
i.e., the amount of CO2 sequestrated. With higher activity, very high and result from the high solubility of minerals
the carbonated SELCHP ash and Kirklees ash show a bearing these elements, such as halite and sylvite. The
lower pH at around 7, compared with only a slight drop release of other elements, with the exception of Cd, Pb
and Zn, was under the limit value for non-hazardous waste.
Figs. 9–11 show the change in the release of Cd, Pb and
14 Zn. Table 4 gives the limit value from the landfill accep-

12 Table 4
Leaching limit values for the acceptance of wastes in landfills (mg/kg)
pH

10 Components Hazardous waste Non-hazardous waste

Cd 1 0.1
8 Cr total 70 10
Cu 100 50
Ni 40 10
6 Pb 50 10
EA FA1 EA FA2 EA FA3 SE FA CL FA KI FA
Zn 200 50
untreated carbonated Cl 25,000 15,000
SO4 50,000 20,000
Fig. 6. pH value of original and carbonated ash.
 X. Li et al. / Waste Management 27 (2007) 1200–1206 1205

500 10000

400 1000
chloride (g/kg)

Lead (mg/kg)
300 100

200 10

100 1

0 0.1
EA FA1 EA FA2 EA FA3 SE FA CL FA KI FA EA FA1 EA FA2 EA FA3 SE FA CL FA KI FA
uncarbonated carbonated uncarbonated carbonated

Fig. 7. Chloride leaching from FA. Fig. 10. Lead leaching.

10000
16000
1000
sulfate (mg/kg)

12000

Zinc (mg/kg)
100
8000
10
4000
1
0
EA FA1 EA FA2 EAFA3 SE FA CL FA KI FA 0.1
uncarbonated carbonated EA FA1 EA FA2 EA FA3 SE FA CL FA KI FA
uncarbonated carbonated
Fig. 8. Sulfate leaching from FA.
Fig. 11. Zinc release.

tance criteria. Cadmium release from the majority of ashes

was lower than the limit value for hazardous wastes (1 mg/ 2–3 orders of magnitudes to lower than 6 mg/kg, with the
kg) but higher than that for non-hazardous wastes (0.1 mg/ exception of CL FA.
kg). No obvious relationship between carbonation and The change of zinc mobility is more complicated. The
cadmium mobility was found. However, it should be noted zinc release from the carbonated aged ash provided by
that for the fly ashes SE FA and KI FA, leachable Cd the Environment Agency was reduced by 1–2 orders of
increased after carbonation. This may result from an magnitude. For the fresh ashes, some showed only slight
increase in cadmium solubility at lower pH (West General change, and in KI FA the zinc is mobilized after
Incorporated). carbonation.
Lead release from all of the ashes decreased dramatically
with carbonation. The lead concentration in almost all of 4. Conclusions
the untreated ash leachate was much higher than the land-
fill acceptance value, but after carbonation it was reduced The fly ash could combine with 7–10% w/w of carbon
dioxide during accelerated carbonation. Water/solid ratio
and temperature have a major influence on the reaction.
As water is a medium of dissolution, ionization and trans-
100 portation of CO2, very low or very high water ratios will
retard the reaction. High temperatures will improve the
10 reaction velocity but do not improve the sequestration of
Cadium (mg/kg)

CO2 into the ash. The carbonation reaction is optimum

1 at ambient temperatures and a water solids ratio of 0.3.
Mineralogical analysis of ash by XRD showed the dis-
0.1 appearance of lime/portlandite and the formation of car-
bonate salts, such as calcite and vaterite upon
0.01 carbonation. With ash from the Kirklees incinerator,
EA FA1 EA FA2 EAFA3 SE FA CL FA KI FA
hydration occurred as well as carbonation. The pH of nat-
urally aged ashes collected from the Environment Agency
uncarbonated carbonated
is near 12, slightly lower than the fresh ashes, which were
Fig. 9. Cadmium leaching. a pH of 12–12.5. The pH was reduced to between 7 and
1206 X. Li et al. / Waste Management 27 (2007) 1200–1206

9, depending on the reactivity of ash. Therefore, carbon- Johnson, D.C., MacLeod, C.L., Hills, C.D., 2003. Solidification of
ation has a significant neutralizing effect on fly ash. stainless steel slag by accelerated carbonation. Environmental Tech-
nology 24, 671–678.
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carbonation. The reaction also led to a significant reduc- systems by CO2. Cement and Concrete Research 2, 567–576.
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increased. It can be concluded that carbonation can reduce Mangialardi, T., 2001. Sintering of MSW fly ash for reuse as a concrete
the hazardous nature of fly ash, and facilitate disposal and aggregate. Journal of Hazardous Materials B87, 225–239.
Mangialardi, T., 2003. Disposal of MSWI fly ash through a combined
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