Sustain. Environ. Res., 20(6), 381-385 (2010)
(Formerly, J. Environ. Eng. Manage.)
381
A CASE STUDY OF THE MICROWAVE SINTERING FOR THE STABILIZATION
OF MSWI FLY ASH
Ching-Lung Chen,1 Shang-Lien Lo,1,* Wen-Hui Kuan2 and Ching-Hong Hsieh1
1
Graduate Institute of Environmental Engineering
National Taiwan University
Taipei 106, Taiwan
2
Department of Safety Health and Environmental Engineering
MingChi University of Technology
Taipei 243, Taiwan
Key Words: Fly ash, stabilization, microwave sintering, lead
ABSTRACT
Municipal solid waste incineration fly ash is generated in a significant amount; it is classified
as hazardous waste in Taiwan. In order to reduce the volume of treated fly ash, a sintering
technology can be considered. However, a traditional sintering consumes too much energy because
of the limit of heat transfer. Hence, a microwave process which can provide quick, uniform, and
selective heating was considered to substitute a traditional thermal process in the sintering
technology. In this study a traditional sintering with electro-furnace at 800, 900, 1000, 1100 and
1200 °C for 30 min and the microwave sintering at 1000 W for 15, 20, 25 and 30 min were
performed. The results indicated that the microwave sintering had better sintering efficiency than a
traditional sintering, and the former could stabilize and transform washed fly ash with calcium
carbonate into blocks in a short time at 1 kW.
INTRODUCTION
Municipal solid waste (MSW) incineration
(MSWI) fly ash is generated in a significant amount; it
is classified as hazardous waste in Taiwan. The usual
treatment method is cement solidification, but it has a
disadvantage of increase in waste volume. Hence,
other treatment methods, such as traditional fusion
and sintering, should be considered. Wainwright and
Cresswell [1] indicate that it is possible to successfully manufacture synthetic lightweight aggregates
from the combustion ashes derived from the incineration of sewage sludge, MSW and pulverised coal.
Karamanov et al. [2] indicate that a mixture, which
consists of MSW ashes and waste from feldspar production, can be transformed into non porous glassceramics at a 30 °C min-1 heating rate and a 40 min
isothermal step at 1120 °C, near the liquidus temperature. Aloisi et al. [3] indicate that a glass ceramic
composite can be obtained by sinter-crystallisation of
vitrified MSW bottom ashes with the addition of alumina waste. Chiou et al. [4] use sewage sludge ash as
the principal material and sewage sludge as the admixture to sinter lightweight aggregate. Wu et al. [5,6]
*Corresponding author
Email: [email protected]
transform water treatment sludge into Al-containing
adsorbents by sintering. The above-mentioned studies
show that traditional fusion and sintering technologies
can not only avoid the shortcoming of cement solidification but also transform powdery waste into all kinds
of products like aggregate, glass ceramics and adsorbent. However, traditional fusion and sintering technologies are not so perfect, they consume much energy because of limit of heat transfer.
Tai and Jou [7] indicate that it is technically feasible to stabilize chromium in soil by the application
of granular activated carbon or iron weirs with microwave radiation energy. Gan [8] indicates that microwave radiation can be used for detoxication of the
sediment sludge through microwave heating, drying
and metal ion immobilization within the sediment solids. Menéndez et al. [9] indicate that the temperature
of sewage sludge with a small amount of microwave
absorber, which is the char produced in the pyrolysis
itself, can be raised up to 900 °C by microwave heating in short tome. This situation results in quick drying and pyrolysis for sewage sludge. Others [10-13]
have used various microwave processes with some
additives to achieve the drying and stabilization of the
Sustain. Environ. Res., 20(6), 381-385 (2010)
382
acid-extracted industrial sludge. These studies show
that microwave energy can be used for the treatments
of swage sludge, contaminated soil and industrial
sludge because microwave can provide quick, uniform
and selective heating. Hence, a microwave process
was considered to substitute traditional heating for a
sintering technology, and this microwave sintering
could stabilize and transform fly ash into blocks at the
same time.
This study aims to establish a microwave sintering which stabilizes and transforms MSWI fly ash into
blocks. The stabilization effect of a sinter was evaluated from the results of the Toxicity Characteristic
Leaching Procedure (TCLP) test and the modified
TCLP test. And a traditional sintering with an electrothermal furnace was also performed to compare with
the microwave sintering for sintering efficiency.
EXPERIMENTAL METHODS AND
MATERIALS
The fly ash was gathered from a MSWI in north
Taiwan. Dried fly ash of 150 g was mixed with water
of 1500 mL and the mixture was rotated at 30 rpm for
40 min. Afterward, fly ash and the above solution
were separated. The washed fly ash underwent mixing,
rotation, and separation processes again, which resulted in that the ratio of total solution to solid was
20:1. Two batches of the solution were collected together and the pH of the collective solution was adjusted to 7.0 ± 0.2 with nitric acid. Afterwards, sodium carbonate of 160 g was added to the adjusted solution to recover calcium ions in the form of calcium
carbonate. Washed fly ash and recovered calcium carbonate were dried at 105 °C and then mixed well. 8 g
washed fly ash with calcium carbonate (called CFA)
was pressed by hands with a set of simple equipment
(a stainless steel bar and ring) into a pellet, of which
the diameter and height were 2 cm and about 4 cm, respectively. CFA was undergone a microwave digestion with 3 mL H2O2, 1.5 mL, HCl, 4.5 mL HNO3 and
3 mL HF with the following conditions: (1) the temperature of the digestion solution rose from room
temperature to 150 °C at 600 W for 20 min; (2) temperature rose again from 150 to 180 °C at 800 W for
10 min; and (3) temperature kept at 180 °C at 800 W
for 20 min.
tion about 0.3 cm away from the crucible opening (Fig.
1). A series of microwave processes at 1 kW for 15,
20, 25 and 30 min were performed with air purging
into microwave oven. At the end of pre-set time, the
crucible was cooled in the microwave oven, and then
the crucible was moved to a ventilation system to cool
down completely. The sinter was crashed in a mortar
and sieved with mesh No. 14 (1 mm). 1.0 g powder
passed through the sieve were taken for TCLP test
with the extraction solution B (20 mL 0.1 N acetic
acid solution, pH = 2.88 ± 0.05). Another 1 g powder
was subject to the modified TCLP test as: (i) 16 mL
0.125 N acetic acid solution of 16 mL was added and
the mixture of powder and acetic acid solution was rotated at 30 rpm for 24 h; (ii) during the rotation period,
the pH of the mixture was controlled at 5.0 ± 0.2 with
nitric acid; (iii) accumulated volume of the mixture
was increased to 20 mL with deionized water after 1 h;
and (iv) mixture was filtered after rotation. CFA with
no treatment also was subject to the TCLP test and the
modified TCLP test for comparison. The digestion solution and all filtrates were analyzed with an inductively coupled plasma, JY24.
2. Traditional Sintering with an Electro-thermal
Furnace
After the temperature of the electro-thermal furnace was raised to designated degree (800-1200 °C), a
crucible with a CFA pellet of 8.0 g was placed into the
furnace for sintering process for 30 min. After each
sintering process was finished, the crucible was removed from the furnace to a ventilation system to cool
its temperature down.
RESULTS AND DISCUSSION
Silicate which was a major component in washed
fly ash does not absorb microwave energy except at
very high temperature. Hence, recovered calcium carbonate which is able to absorb microwave energy and
transform it into heat at comparatively low temperature played a role of the microwave absorber in the
pellet. In addition, the purpose of the carbonate lining
Fireproof material (FM)
Aluminum
oxide cake
1. Microwave Sintering
A piece of graphite was laid on an aluminum oxide cake (18 g aluminum oxide and 2 g gypsum) on
the bottom of a crucible (15 mL). Then, a lining of the
mixture of powdered activated carbon (PAC) and gypsum with the ratio of 3:1 was placed on the crucible
wall.
After a CFA pellet was put into a modified crucible, a piece of fireproof material was set at the posi-
PAC lining
CFA pellet
FM
FM
Graphite
Baseplate
Fig. 1. The structure of the modified crucible on the
microwave baseplate
Chen et al.: Microwave Sintering MSWI Fly Ash
(a)
(b)
383
Table 1. The TCLP test result of MSWI fly ash
Element
Cd
Cr
Cu
Ni
Concentration
a
0.63 1.04
0.54
(mg L-1)
a
the concentration was lower than 0.1 mg L-1
(c)
(d)
in the modified crucible was also to absorb and transform microwave energy into heat to raise the temperature in the crucible to the degree that calcium carbonate can start to absorb microwave energy. Hence, the
thermal mechanism in the microwave sintering is that:
(i) the carbonate lining absorbs and transforms microwave energy into heat at room temperature; (ii)
heat from the carbonate lining raises the temperature
of the pellet; (iii) temperature of the pellet is high
enough to let calcium carbonate absorb and transform
microwave energy into heat; (iv) temperature of the
pellet rises continuously and achieves the degree
which silicate starts to absorb and transform microwave energy into heat; and (v) temperature of the pellet is high enough to sinter itself.
Figure 2 shows the shapes of CFA pellets with
no treatment, after a traditional sintering at 1200 °C
for 30 min, and after the microwave sintering for 15
and 30 min. In a traditional sintering, when the furnace temperature below 1100 °C, CFA pellets were
not sintered and easy to be broken by hands. When the
temperature of the furnace was 1200 °C, the pellet
seemed to be sintered, but the top one third of it still
could be cracked easily. The shape of a pellet after the
microwave process for more than 15 min shrank, due
to the fact that the pressure for pressing CFA into a
pellet was not high. All the sinters made by the microwave sintering for 15 to 30 min could not be
cracked by hands. These results show that the temperature of the electro-thermal furnace needed to be
higher than 1200 °C to get a sinter by a traditional sintering as hard as that by the microwave sintering. Because the maximum power of a 1200 °C furnace is 2.5
kW, it is certain that the sintering efficiency of the microwave sintering is better than that of a traditional
sintering from the viewpoint of energy consumption.
Table 1 shows the leaching result of MSWI fly
Zn
54.2
4.28
Table 2. The element compositions of CFA
Element
Fig. 2. The shapes of CFA pellets treated by different
sintering technologies. (a) CFA pellet with no
treatment; (b) CFA pellet after a traditional
sintering at 1200 °C for 30 min; (c) CFA pellet
after a microwave sintering at 1 kW for 15 min;
(d) CFA pellet after a traditional sintering at 1
kW for 30 min.
Pb
Al
Ba
Ca
Cd
Cr
Cu
Fe
K
Concentration
(mg g-1)
28.5
14.3
254
0.59
0.67
1.01
8.10
9.62
Element
Mg
Mn
Na
Ni
Pb
Sr
Zn
Concentration
(mg g-1)
9.66
1.23
29.2
1.61
3.86
2.25
21.4
ash after the TCLP test. The leaching concentration of
lead ions from fly ash was 54.2 mg L-1, and those of
other heavy metal ions were lower than the TCLP
regulatory limits. Table 2 shows the result of CFA after microwave digestion. It was found that the amount
of lead was the highest among all heavy metals form
TCLP fly ash data (Table 1). Hence, the leaching concentration of lead ions was selected as a stabilization
index in this study.
Figure 3a shows the pH values and leaching
concentrations of lead ions from CFA and sintered
CFA after the TCLP test. It was found that the leaching concentrations of lead ions from both sinters and
CFA were lower than 5 mg L-1. Compared the result
of the TCLP test of CFA with that of MSWI fly ash, it
seems that a wash process for fly ash treatment is
enough to reduce its leaching problem, and the microwave sintering was unnecessary except for block
formation. However, it is noticed that the pH of CFA
after the TCLP test is 7.1 and those of sinters made by
the microwave sintering for 15 to 30 min was about
12.2. The study of Meima and Comans [14] indicates
that lead ions in fly ash may be transformed to the stable compound of chloropyromorphite (Pb5(PO4)3Cl)
when the pH of the extraction solution is close to 7.
When the pH of the extraction solution is high than 12,
the lead ions are released gradually with the increase
in pH. However, in this study the leaching concentration of lead ions from each sinter was very low, and
the possible reason was that released lead ions were
combined with additional carbonate anions and transformed to lead carbonate (Ksp = 1.5 × 10–13), or lead
ions were stabilized indeed in the sinter matrix.
In order to further check the stabilization effects
of CFA and sinters made by the microwave sintering,
the modified TCLP test was performed. Figure 3b
shows the final pH values and the leaching concentrations of lead ions from CFA and sinters after the
modified TCLP test. The final pH value of each sam-
Sustain. Environ. Res., 20(6), 381-385 (2010)
be performed in the future study.
1
14
(a)
ACKNOWLEDGEMENTS
12
0.8
Leaching variation(Sintered CFA)
0.4
Leaching conc. (CFA)
pH variation (Sintered CFA)
pH (CFA)
8
6
pH
10
0.6
REFERENCES
2
0
0
100
(b)
80
Leaching variation (Sintered CFA)
Leaching conc. (CFA)
pH variation (Sintered CFA)
pH (CFA)
60
14
12
10
8
6
40
4
20
0
2
0
10
20
Sintering time (min)
This work was financially supported by the Ministry of Economic Affairs, R.O.C. in the framework of
the projects 97-EC-17-A-10-S1-0007.
4
0.2
pH
Lead ion conc. in the leachate (mg L-1) Lead ion conc. in the leachate (mg L-1)
384
30
0
Fig. 3. The extraction results of CFA before and after
the microwave sintering: (a) TCLP; (b) modified
TCLP.
ple was close to 5.0, so the most part of lead ions in
each sample should be released. The leaching concentration of lead ions from CFA was 82 mg L-1 and all
those from sinters were lower than 5 mg L-1. These results showed that the CFA stabilization displayed in
the result of the TCLP test was a temporary phenomenon because of the extraction solution at a suitable pH
value, and the microwave sintering indeed stabilized
CFA pellets. Hence, the microwave sintering was necessary for the stabilization of the washed fly ash. The
possible stabilization mechanism was that lead ions
were wrapped in the sinter matrix and the matrix
around lead ions prevented them from contact of the
extraction situation. Hence, the leaching concentration
of lead ions from the sinter decreased substantially.
From the results of the sintering comparison and the
modified TCLP test, it is certain that the microwave
sintering could stabilize and transform a CFA pellet
into block in a short time.
CONCLUSIONS
From the results in this study, it is demonstrated
that the microwave sintering has a better sintering efficiency than a traditional sintering, and the former
could stabilize and transform washed fly ash with calcium carbonate into block in a short time at the microwave power of 1 kW. Besides, the sinters made by
the microwave process could be used as aggregate,
and the test of compressive strength for them would
1. Wainwright, P.J. and D.J.F. Cresswell, Synthetic
aggregates from combustion ashes using an
innovative rotary kiln. Waste Manage., 21(3),
241-246 (2001).
2. Karamanov, A., M. Pelino and A. Hreglich,
Sintered glass-ceramics from municipal solid
waste-incinerator fly ashes-part I: The influence of
the heating rate on the sinter-crystallisation. J. Eur.
Ceram. Soc., 23(6), 827-832 (2003).
3. Aloisi, M., A. Karamanov, G. Taglieri, F. Ferrante
and M. Pelino. Sintered glass ceramic composites
from vitrified municipal solid waste bottom ashes.
J. Hazard. Mater., 137(1), 138-143 (2006).
4. Chiou, I.J., K.S. Wang, C.H. Chen and Y.T. Lin.
Lightweight aggregate made from sewage sludge
and incinerated ash. Waste Manage., 26(12),
1453-1461 (2006).
5. Wu, C.H., C.F. Lin and P.Y. Horng, Adsorption of
copper and lead ions onto regenerated sludge from
a water treatment plant. J. Environ. Sci. Heal. A,
39(1), 237-252 (2004).
6. Wu, C.H., C.F. Lin and P.Y. Horng, Regeneration
and reuse of water treatment plant sludge:
Adsorbent for cations. J. Environ. Sci. Heal. A,
39(3), 717-728 (2004).
7. Tai, H.S. and C.J.G. Jou, Immobilization of
chromium-contaminated soil by means of
microwave energy. J. Hazard. Mater., 65(3), 267275 (1999).
8. Gan, Q., A case study of microwave processing of
metal hydroxide sediment sludge from printed
circuit board manufacturing wash water. Waste
Manage., 20(8), 695-701 (2000).
9. Menéndez, J.A., M. Inguanzo and J.J. Pis,
Microwave-induced pyrolysis of sewage sludge.
Water Res., 36(13), 3261-3264 (2002).
10. Chen, C.L., S.L. Lo, W.H. Kuan and C.H. Hsieh,
Stabilization of Cu in acid-extracted industrial
sludge using a microwave process. J. Hazard.
Mater., 123(1-3), 256-261 (2005).
11. Chen, C.L., S.L. Lo, P.T. Chiueh, W.H. Kuan and
C.H. Hsieh, The assistance of microwave process
in sludge stabilization with sodium sulfide and
Chen et al.: Microwave Sintering MSWI Fly Ash
sodium phosphate. J. Hazard. Mater., 147(3), 930937 (2007).
12. Hsieh, C.H., S.L. Lo, P.T. Chiueh, W.H. Kuan and
C.L. Chen, Microwave enhanced stabilization of
heavy metal sludge. J. Hazard. Mater., 139(1),
160-166 (2007).
13. Hsieh, C.H., S.L. Lo, C.Y. Hu, K. Shih, W.H.
Kuan and C.L. Chen, Thermal detoxification of
hazardous
metal
sludge
by
applied
electromagnetic energy. Chemosphere, 71(9),
1693-1700 (2008).
14. Meima, J.A. and R.N.J. Comans, The leaching of
385
trace elements from municipal solid waste
incinerator bottom ash at different stages of
weathering. Appl. Geochem., 14(2), 159-171
(1999).
Discussions of this paper may appear in the discussion section of a future issue. All discussions should
be submitted to the Editor-in-Chief within six months
of publication.
Manuscript Received: October 31, 2008
Revision Received: January 28, 2009
and Accepted: February 10, 2009