- Email: [email protected]
Treatment and recycling of incinerated ash using thermal plasma technology
Waste Management 22 (2002) 485–490 www.elsevier.com/locate/wasman
Treatment and recycling of incinerated ash using thermal plasma technology T.W. Chenga,*, J.P. Chub, C.C. Tzengc, Y.S. Chena a
Department of Materials & Mineral Resources Engineering, National Taipei University of Technology, Taipei, Taiwan, Republic of China b Institute of Materials Engineering, National Taiwan Ocean University, Keelung, Taiwan, Republic of China c Physics Division, Institute of Nuclear Energy Research, Lung-Tan, Taiwan, Republic of China Accepted 19 June 2001
Abstract To treat incinerated ash is an important issue in Taiwan. Incinerated ashes contain a considerable amount of hazardous materials such as dioxins and heavy metals. If these hazardous materials are improperly treated or disposed of, they shall cause detrimental secondary contamination. Thermal plasma vitrification is a robust technology to treat and recycle the ash residues. Under the high temperature plasma environment, incinerated ashes are vitrified into benign slag with large volume reduction and extreme detoxification. Several one-step heat treatment processes are carried out at four temperatures (i.e. 850, 950, 1050 and 1150 C) to obtain various ‘‘microstructure materials’’. The major phase to form these materials is a solid solution of gehlenite (Ca2Al2SiO7) and a˚kermanite (Ca2MgSi2O7) belonging to the melilite group. The physical and mechanical properties of the microstructure materials are improved by using one-step post-heat treatment process after plasma vitrification. These microstructure materials with good quality have great potential to serve as a viable alternative for construction applications. # 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction Disposal of large quantities of municipal solid waste is a major problem in Taiwan. Incineration has become an important treating method for combustible solid wastes, especially in municipal areas due to the increasing difficulty to get suitable sites for traditional landfill. Currently, there are five municipal waste incinerators with capacity of more than 300 t per day, and 29 small medical waste incinerators are in operation in Taiwan [1]. On the other hand, there are 22 municipal waste incinerators under construction in Taiwan that are expected to be complete at the end of 2003. It is estimated that these incinerators will produce over two million tonnes of incinerated ashes every year when these new incinerators are all put into operation. Fly ashes from municipal incinerators are classified as hazardous materials in Taiwan. With growing public concerns and rigorous regulatory requirements, hazar* Corresponding author. Tel.: +886-2-27712171, ext. 2730; fax: +886-2-27317185. E-mail address: [email protected] (T.W. Cheng).
dous waste disposal practices being used to date offer challenges. For example, the ash residues after incineration of municipal solid wastes needs further treatment such as consolidation before it becomes reasonably safe for the environment. Therefore, a viable competing immobilization technology is required. This technology should maximize safety factors and reliability to transfer the ashes into a stable form. Our prior studies on mixed medical waste vitrification [2] have shown that thermal plasma is promising technology for hazardous waste treatment. In thermal plasma vitrification, the heat generated from plasma is used to process hazardous wastes containing metals, inorganic oxides and/or organics at temperatures above 1500 C. Metalbearing wastes are melted and organic contaminants are thermally destroyed. The plasma vitrification yields a glass-like, leach-resistant monolith slag, which is environmentally safe for landfill disposal and/or can be reused as glass-ceramic for construction materials, such as interior and exterior wall cladding or ordinary floor tile applications. Incinerated ash, containing large amount of silica, can be a good candidate for glass-ceramic production.
0956-053X/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0956-053X(01)00043-5
486
T.W. Cheng et al. / Waste Management 22 (2002) 485–490
Glass-ceramic materials are polycrystalline solids consisting of the residual glass phase. The variety of crystalline phase and the properties of the glass-ceramic, including the number of crystals for nucleation, the growth rate and the microstructure, can be controlled by the initial composition and by suitable post-heat treatment. Recent investigations have reported on the use of a molten and crystallization process to recycle different waste materials. These waste materials included, for example, red mud from zinc hydrometallurgy [3], coal fly ash [4–6], blast furnace slags [7,8], industrial and mining wastes [9], incinerated ashes [10,11]. Thus, high-temperature molten technology is one of potential technologies for engineering applications. In the present study, a lab-scale plasma system built at Institute of Nuclear Energy Research (INER) exclusively for the treatment of hazardous/radioactive wastes was used for vitrification processes. This study was directed toward characterization of post-heat treatment for slag generated from plasma technology, in order to establish a better understanding of the feasibility of the INER plasma system in treating the incinerated ashes. The microstructure materials were characterized by toxicity characteristic leaching procedure (TCLP) for testing the leaching resistance, X-ray diffractometry (XRD) for crystal structure determination, scanning electron microscopy (SEM) for microstructure/morphology observation, and energy dispersive spectroscopy (EDS) for X-ray chemical microanalysis. In addition, other properties, such as microhardness, porosity, water absorption and bulk specific gravity were also evaluated.
2. Experimental procedures The incinerated ash used in this investigation was obtained from one of the municipal solid waste incinerators in Taipei. Table 1 shows the chemical composition of the incinerated ash. A 100 kW non-transferred plasma torch was used as the heat source in the INER lab-scale plasma system. Argon was the plasma gas for ignition, and nitrogen was served as the carrier gas
during the treatment. A graphite crucible containing about 2400 g samples was placed in the center of the chamber so that a uniform temperature distribution could be achieved in the crucible. The heating rates of ash samples were controlled to about 9 C/min. The molten slag was kept at 1350 C for 10 min, and then cooled at a cooling rate less than 10 C/min to desired temperatures for the post-heat treatment. The molten slag was post-heat treated for 2 h at temperatures of 1150, 1050, 950, and 850 C, and then cooled to room temperature. The microstructure material thus formed is cut into small pieces by a diamond saw for material characterizations. For SEM and EDS examinations, a Hitachi S-4100 scanning electron microscope was used to examine the microstructure materials that were polished and etched by 2% HF for 1 min. XRD was done with a Rigaku D/MAX-VB diffractometer with CuKa radiation.
3. Results and discussions 3.1. Volume and weight reductions Table 2 shows a list of volume and weight reduction for microstructure materials in this study. The incinerated ash feeds in general have large volume reduction ratios (average 6.4) after the plasma molten treatment. Nevertheless, the weight reduction ratio is about 1.7 on average. This could be due to the fact that the incinerated ash had less combustible or organic constituents. TCLP testing results of the microstructure materials obtained in this study are also given in Table 2. Each sample analyzed has insignificant leachability characteristics for the Cr, Pb, and Cd. As will be discussed subsequently, the low leachability is closely related to the slag structures. 3.2. EDS chemical analysis Elemental analysis results of the microstructure materials by X-ray energy dispersive spectrometer are presented in Table 3. It is shown that major elemental components of microstructure materials are, in order of Table 2 List of post treatment condition, volume and weight reduction ratios and TCLPa analysis results of microstructure materials
Table 1 The major chemical composition of incinerated ash Chemical composition
Wt.%
CaO SiO2 Al2O3 TiO2 Fe2O3 P2O5 Na2O K2O MgO
32.96 12.41 8.06 2.18 2.35 1.92 5.15 2.41 2.23
Treated temperature ( C) 850 950 1050 1150 a
Volume reduction
6.28 5.53 5.77 7.82
Weight reduction
1.71 1.69 1.72 1.81
Microstructure material leached (mg/l) Cr
Pb
Cd
N/A N/A N/A N/A
N/A N/A N/A N/A
0.04 0.04 0.04 0.04
TCLP, toxity characteristic leaching procedure.
487
T.W. Cheng et al. / Waste Management 22 (2002) 485–490
amount measured, calcium, silicon, aluminum, sodium, magnesium, iron, titanium, and phosphorous, with trace amounts of carbon, sulfur, chromium, zinc, manganese, and chlorine present. Except for the sulfur, the effect of heat-treatment temperature was not evident, suggesting that the post-heat treatment had no apparent effects in varying the major slag chemical composition. High sulfur contents in the slag treated at low temperatures implied that the high-temperature thermal treatment is needed for the removal of sulfur. 3.3. XRD phase identification In order to identify the crystalline phases, XRD was carried out on the microstructure materials after the heat treatment at 1150, 1050, 950, and 850 C. XRD results are shown in Fig. 1. The Joint Committee of Powder Diffraction Standard (JCPDS) card shows that the reflections are mainly assigned to two members of the melilite group—gehlenite (Ca2Al2SiO7) and a˚kermanite
(Ca2MgSi2O7), with small deviations of the 1-values. Gehlenite and a˚kermanite have the same crystal structure and their lattice parameters are very close to each other. Therefore, the crystallizing phases could transform essentially into a solid solution of gehlenite and a˚kermanite. A similar result was also reported for a slag-based glass-ceramic in Turkey [8]. The fundamental unit on the structure of silicates basically consists of four O 2 at the apices of a regular tetrahedron surrounding and coordinated by one Si+4 at the center. The strength of any single Si–O bond is just equal to one-half of the total bonding energy available in the oxygen ion. Every O 2 has the potentiality of bonding to another silicon ion and entering into another tetrahedral grouping, thus uniting the tetrahedral groups through the shared oxygen, and therefore forming the great variety of silicate structures. The low leachability characteristics for Cr, Pb, and Cd thus presumably were because the heavy metal ions replaced parent ions (Al+3 and Ca+2 in this study) and enclosed in the framework of silicates.
Table 3 EDSa results for the microstructure materials after heat treatments at various temperatures Element wt.% ( C)
C
S
Cr
Mn
Zn
Cl
Si
K
Na
Mg
Al
P
Ca
Ti
Fe
850 950 1050 1150
0.01 0.01 0.02 0.01
0.19 0.13 0.09 0.07
0.13 0.11 0.12 0.10
0.15 0.13 0.14 0.13
0.40 0.58 0.86 0.46
0.05 0.05 0.05 0.01
14.30 13.40 14.30 14.00
0.72 0.70 0.59 0.67
2.96 2.81 2.77 2.85
2.68 2.64 2.70 2.76
8.00 7.96 7.29 8.05
1.44 1.58 1.51 1.51
25.00 24.20 23.40 23.50
1.53 1.71 1.70 1.71
2.05 2.42 2.32 2.21
a
EDS, energy dispersive spectroscopy.
Fig. 1. X-ray diffractometry (XRD) results of the microstructure materials after heat treatments at different temperatures for 2 h.
488
T.W. Cheng et al. / Waste Management 22 (2002) 485–490
Fig. 1 also shows that the Bragg peak intensities were low after heat treatment for 2 h at 1150 C. This is presumably resulted from the relatively small driving force for the phase transformation. At this temperature, the atomic diffusion rate was high whereas the amounts of nucleation and crystallization were small. When treated at low temperatures such as 1050 or 950 C, the driving force for the phase transformation increased because more nuclei were obtained. When treated at 850 C, the driving force increased extensively while the atomic diffusion rate was low, resulting in formation of fewer new phases. 3.4. SEM microstructural examinations SEM microstructural examinations of microstructure materials after heat treatments at different temperatures were performed and the results are shown in Figs. 2–5.
Based on these micrographs, it can be seen that the microstructure materials were not thoroughly homogeneous. In general, the crystal size decreased with decreasing heat-treatment temperature. The average crystal size is around 0.2 mm (Figs. 4 and 5). These results are in a good agreement with XRD results. Generally, the crystallization process at a given temperature is controlled by nucleation and growth rates. Since both nucleation and growth rates vary oppositely with the temperature, it is expected that those two curves for nucleation and growth rates will overlap in a certain temperature range. Accordingly, in this study optimal temperatures for the heat treatment were identified to achieve better microstructure. When the microstructure materials were heat treated at 1150 or 1050 C, the crystal growth rate was improved whereas the crystallization was far from optimization due to limited nucleation rates. On the other hand, the heat treatment at low temperatures such as 950 or 850 C yields more
Fig. 2. Scanning electron microscopic (SEM) micrograph of microstructure material sample heat treated for 2 h at 1150 .
Fig. 4. Scanning electron microscopic (SEM) micrograph of microstructure material sample heat treated for 2 h at 950 C.
Fig. 3. Scanning electron microscopic (SEM) micrograph of microstructure material sample heat treated for 2 h at 1050 C.
Fig. 5. Scanning electron microscopic (SEM) micrograph of microstructure material sample heat treated for 2 h at 850 C.
T.W. Cheng et al. / Waste Management 22 (2002) 485–490
nuclei as well as an increased nucleation rate, thus producing a better and refined crystallization in materials microstructure (Figs. 4 and 5). Therefore, an improved, refined gehlenite microstructure material could be attained after the heat treatment at 850 or 950 C by using thermal plasma technology. 3.5. Physical and mechanical properties Various physical and mechanical properties of microstructure materials obtained from plasma vitrification with post-heat treatment at different temperatures are listed in Table 4. Densities of the microstructure materials were in the range of 2.9–3.0 g/cm3. Because these values are comparable with that of melilite (2.95 to 3.0 g/cm3) [12], the melilite as a major crystalline phase in the microstructure materials is then confirmed, and consistent with XRD results. The hardness properties were measured for feasibility evaluation of the materials for a given purpose. Hardness results shown in Table 4 reveal that the microstructure materials had Knoop hardness of 4.5–4.8 GPa and Mohs’ scale of 6.5–7. Thermal expansion is also one of important properties. As listed in Table 4, relatively low thermal-expansion coefficients suggest the microstructure materials obtained are suitable for the high-temperature applications. Furthermore, preferred evaluation results of other physical properties (water adsorption, porosity, and machineability) in Table 4 suggest the microstructure materials were fairly dense and thus may be useful for construction applications. It is interesting to note that many properties such as hardness, porosity, and water adsorption are improved with decreasing the heattreatment temperature. The improved properties are presumably attributed to the refined structure achieved in the low-temperature treated sample.
4. Conclusions Treatment and characterizations of the incinerated ash by using thermal plasma technology have been carried out. After post-heat treatments at different temperatures, microstructure materials with good quality Table 4 Various properties of the microstructure materials Properties
850 C
950 C
1050 C
1150 C
Density (g/cm3) Mohs’ hardness Knoop hardness (GPa) Porosity (%) Water absorption (%) Thermal expansion coefficient (10 6 mm C; 25–450 C) Machinability
2.99 7.0 4.84 0.83 0.28 9.85
2.96 7.0 4.70 1.81 0.61 8.21
2.94 6.5 4.62 2.68 0.91 8.68
2.93 6.5 4.57 2.31 0.79 8.41
Good
Good
Good
Good
489
are obtained. After plasma vitrification and one-step post-heat treatment, large volume and weight reduction ratios (6.4 and 1.7, respectively) are achieved. Heavy metals originated in the incinerated ash are confined in the treated silicate framework, hence yielding very low leachability results. Therefore, thermal plasma technology can be an effective method to treat incinerated ashes. After the high temperature melting and heat treatment process, microstructure materials were formed and consisted of predominantly a melilite group solid solution of gehlenite and a˚kermanite. It was found that the crystallization was progressively improved as the post-heat treatment temperature was decreased. The average grain size of the microstructure materials was 0.2 mm at the heat treatment temperature of 850– 950 C. Microstructure materials with improved physical and mechanical properties were thus obtained after being heat treated at low temperatures. Microstructure materials with preferred properties therefore have the potential to serve as a viable alternative for construction applications.
Acknowledgements The National Science Council of Republic of China supported this study under contract NSC-89-2211-E027-003, which was gratefully acknowledged.
References [1] Ling YC, Hou PCC. A Taiwanese study of 2,3,7,8-substituted PCDD/DFs and coplanar PCBs in fly ashes from incinerators. Journal of Hazardous Materials 1998;58:83–91. [2] Chu JP, Hwang IJ, Tzeng CC, Kuo YY, Yu YJ. Characterization of vitrified slag from mixed medical waste surrogates treated by a thermal plasma system. Journal of Hazardous Materials 1998;58: 172–92. [3] Romero M, Rinco´n J Ma. Preparation and properties of high iron oxide content glasses obtained from industrial wastes. Journal of the European Ceramic Society 1998;18:153–60. [4] Boccaccini AR, Bu¨cker J, Bossert J. Glass and glass-ceramaics from coal fly-ash and waste glass. Tile & Brick 1996;12(6):515–8. [5] Barbieri L, Manfredini T, Queralt I, Rinco´n J Ma, Romero M. Vitrification of fly ash from thermal power station. Glass Technology 1997;38(5):165–70. [6] Barbieri L, Lancellotti I, Manfredini T, Queralt I, Rinco´n J Ma, Romero M. Design, obtainment and properties of glasses and glass-ceramics from coal fly ash. Fuel 1999;78:271–6. [7] Rawlings RD. Production and properties of silceram glass-ceramic. Glass-ceramic Materials: Fundamentals and Applications. Modena 1997:115–33. [8] O¨vecog˘lu ML. Microstructural characterization and physical properties of a slag-based glass-ceramic crystallized at 950 and 11100 C. Journal of the European Ceramic Society 1998;18:161– 8. [9] Marabini AM, Plescia P, Maccari D, Burragato F, Pelino M. New materials from industrial and mining wastes: glass-ceramics and glass- and rock-wool fibre. International Journal of Mineral Processing 1998;53:121–34.
490
T.W. Cheng et al. / Waste Management 22 (2002) 485–490
[10] Boccaccini AR, Kopf M, Stumpfe W. Glass-ceramics from filter dust from waste incinerators. Ceramics International 1995;21: 231–5. [11] Romero M, Rawlings RD, Rinco´n JMa. Development of a new glass-ceramic by means of controlled vitrification and crystal-
lisation of inorganic wastes from urban incineration. Journal of the European Ceramic Society 1999;19:2049–58. [12] Deer WA, Howie RA, Zussman J. An introduction to the rock-forming minerals. Longman Scientific and Technical, 1992.
Treatment and recycling of incinerated ash using thermal plasma technology T.W. Chenga,*, J.P. Chub, C.C. Tzengc, Y.S. Chena a
Department of Materials & Mineral Resources Engineering, National Taipei University of Technology, Taipei, Taiwan, Republic of China b Institute of Materials Engineering, National Taiwan Ocean University, Keelung, Taiwan, Republic of China c Physics Division, Institute of Nuclear Energy Research, Lung-Tan, Taiwan, Republic of China Accepted 19 June 2001
Abstract To treat incinerated ash is an important issue in Taiwan. Incinerated ashes contain a considerable amount of hazardous materials such as dioxins and heavy metals. If these hazardous materials are improperly treated or disposed of, they shall cause detrimental secondary contamination. Thermal plasma vitrification is a robust technology to treat and recycle the ash residues. Under the high temperature plasma environment, incinerated ashes are vitrified into benign slag with large volume reduction and extreme detoxification. Several one-step heat treatment processes are carried out at four temperatures (i.e. 850, 950, 1050 and 1150 C) to obtain various ‘‘microstructure materials’’. The major phase to form these materials is a solid solution of gehlenite (Ca2Al2SiO7) and a˚kermanite (Ca2MgSi2O7) belonging to the melilite group. The physical and mechanical properties of the microstructure materials are improved by using one-step post-heat treatment process after plasma vitrification. These microstructure materials with good quality have great potential to serve as a viable alternative for construction applications. # 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction Disposal of large quantities of municipal solid waste is a major problem in Taiwan. Incineration has become an important treating method for combustible solid wastes, especially in municipal areas due to the increasing difficulty to get suitable sites for traditional landfill. Currently, there are five municipal waste incinerators with capacity of more than 300 t per day, and 29 small medical waste incinerators are in operation in Taiwan [1]. On the other hand, there are 22 municipal waste incinerators under construction in Taiwan that are expected to be complete at the end of 2003. It is estimated that these incinerators will produce over two million tonnes of incinerated ashes every year when these new incinerators are all put into operation. Fly ashes from municipal incinerators are classified as hazardous materials in Taiwan. With growing public concerns and rigorous regulatory requirements, hazar* Corresponding author. Tel.: +886-2-27712171, ext. 2730; fax: +886-2-27317185. E-mail address: [email protected] (T.W. Cheng).
dous waste disposal practices being used to date offer challenges. For example, the ash residues after incineration of municipal solid wastes needs further treatment such as consolidation before it becomes reasonably safe for the environment. Therefore, a viable competing immobilization technology is required. This technology should maximize safety factors and reliability to transfer the ashes into a stable form. Our prior studies on mixed medical waste vitrification [2] have shown that thermal plasma is promising technology for hazardous waste treatment. In thermal plasma vitrification, the heat generated from plasma is used to process hazardous wastes containing metals, inorganic oxides and/or organics at temperatures above 1500 C. Metalbearing wastes are melted and organic contaminants are thermally destroyed. The plasma vitrification yields a glass-like, leach-resistant monolith slag, which is environmentally safe for landfill disposal and/or can be reused as glass-ceramic for construction materials, such as interior and exterior wall cladding or ordinary floor tile applications. Incinerated ash, containing large amount of silica, can be a good candidate for glass-ceramic production.
0956-053X/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0956-053X(01)00043-5
486
T.W. Cheng et al. / Waste Management 22 (2002) 485–490
Glass-ceramic materials are polycrystalline solids consisting of the residual glass phase. The variety of crystalline phase and the properties of the glass-ceramic, including the number of crystals for nucleation, the growth rate and the microstructure, can be controlled by the initial composition and by suitable post-heat treatment. Recent investigations have reported on the use of a molten and crystallization process to recycle different waste materials. These waste materials included, for example, red mud from zinc hydrometallurgy [3], coal fly ash [4–6], blast furnace slags [7,8], industrial and mining wastes [9], incinerated ashes [10,11]. Thus, high-temperature molten technology is one of potential technologies for engineering applications. In the present study, a lab-scale plasma system built at Institute of Nuclear Energy Research (INER) exclusively for the treatment of hazardous/radioactive wastes was used for vitrification processes. This study was directed toward characterization of post-heat treatment for slag generated from plasma technology, in order to establish a better understanding of the feasibility of the INER plasma system in treating the incinerated ashes. The microstructure materials were characterized by toxicity characteristic leaching procedure (TCLP) for testing the leaching resistance, X-ray diffractometry (XRD) for crystal structure determination, scanning electron microscopy (SEM) for microstructure/morphology observation, and energy dispersive spectroscopy (EDS) for X-ray chemical microanalysis. In addition, other properties, such as microhardness, porosity, water absorption and bulk specific gravity were also evaluated.
2. Experimental procedures The incinerated ash used in this investigation was obtained from one of the municipal solid waste incinerators in Taipei. Table 1 shows the chemical composition of the incinerated ash. A 100 kW non-transferred plasma torch was used as the heat source in the INER lab-scale plasma system. Argon was the plasma gas for ignition, and nitrogen was served as the carrier gas
during the treatment. A graphite crucible containing about 2400 g samples was placed in the center of the chamber so that a uniform temperature distribution could be achieved in the crucible. The heating rates of ash samples were controlled to about 9 C/min. The molten slag was kept at 1350 C for 10 min, and then cooled at a cooling rate less than 10 C/min to desired temperatures for the post-heat treatment. The molten slag was post-heat treated for 2 h at temperatures of 1150, 1050, 950, and 850 C, and then cooled to room temperature. The microstructure material thus formed is cut into small pieces by a diamond saw for material characterizations. For SEM and EDS examinations, a Hitachi S-4100 scanning electron microscope was used to examine the microstructure materials that were polished and etched by 2% HF for 1 min. XRD was done with a Rigaku D/MAX-VB diffractometer with CuKa radiation.
3. Results and discussions 3.1. Volume and weight reductions Table 2 shows a list of volume and weight reduction for microstructure materials in this study. The incinerated ash feeds in general have large volume reduction ratios (average 6.4) after the plasma molten treatment. Nevertheless, the weight reduction ratio is about 1.7 on average. This could be due to the fact that the incinerated ash had less combustible or organic constituents. TCLP testing results of the microstructure materials obtained in this study are also given in Table 2. Each sample analyzed has insignificant leachability characteristics for the Cr, Pb, and Cd. As will be discussed subsequently, the low leachability is closely related to the slag structures. 3.2. EDS chemical analysis Elemental analysis results of the microstructure materials by X-ray energy dispersive spectrometer are presented in Table 3. It is shown that major elemental components of microstructure materials are, in order of Table 2 List of post treatment condition, volume and weight reduction ratios and TCLPa analysis results of microstructure materials
Table 1 The major chemical composition of incinerated ash Chemical composition
Wt.%
CaO SiO2 Al2O3 TiO2 Fe2O3 P2O5 Na2O K2O MgO
32.96 12.41 8.06 2.18 2.35 1.92 5.15 2.41 2.23
Treated temperature ( C) 850 950 1050 1150 a
Volume reduction
6.28 5.53 5.77 7.82
Weight reduction
1.71 1.69 1.72 1.81
Microstructure material leached (mg/l) Cr
Pb
Cd
N/A N/A N/A N/A
N/A N/A N/A N/A
0.04 0.04 0.04 0.04
TCLP, toxity characteristic leaching procedure.
487
T.W. Cheng et al. / Waste Management 22 (2002) 485–490
amount measured, calcium, silicon, aluminum, sodium, magnesium, iron, titanium, and phosphorous, with trace amounts of carbon, sulfur, chromium, zinc, manganese, and chlorine present. Except for the sulfur, the effect of heat-treatment temperature was not evident, suggesting that the post-heat treatment had no apparent effects in varying the major slag chemical composition. High sulfur contents in the slag treated at low temperatures implied that the high-temperature thermal treatment is needed for the removal of sulfur. 3.3. XRD phase identification In order to identify the crystalline phases, XRD was carried out on the microstructure materials after the heat treatment at 1150, 1050, 950, and 850 C. XRD results are shown in Fig. 1. The Joint Committee of Powder Diffraction Standard (JCPDS) card shows that the reflections are mainly assigned to two members of the melilite group—gehlenite (Ca2Al2SiO7) and a˚kermanite
(Ca2MgSi2O7), with small deviations of the 1-values. Gehlenite and a˚kermanite have the same crystal structure and their lattice parameters are very close to each other. Therefore, the crystallizing phases could transform essentially into a solid solution of gehlenite and a˚kermanite. A similar result was also reported for a slag-based glass-ceramic in Turkey [8]. The fundamental unit on the structure of silicates basically consists of four O 2 at the apices of a regular tetrahedron surrounding and coordinated by one Si+4 at the center. The strength of any single Si–O bond is just equal to one-half of the total bonding energy available in the oxygen ion. Every O 2 has the potentiality of bonding to another silicon ion and entering into another tetrahedral grouping, thus uniting the tetrahedral groups through the shared oxygen, and therefore forming the great variety of silicate structures. The low leachability characteristics for Cr, Pb, and Cd thus presumably were because the heavy metal ions replaced parent ions (Al+3 and Ca+2 in this study) and enclosed in the framework of silicates.
Table 3 EDSa results for the microstructure materials after heat treatments at various temperatures Element wt.% ( C)
C
S
Cr
Mn
Zn
Cl
Si
K
Na
Mg
Al
P
Ca
Ti
Fe
850 950 1050 1150
0.01 0.01 0.02 0.01
0.19 0.13 0.09 0.07
0.13 0.11 0.12 0.10
0.15 0.13 0.14 0.13
0.40 0.58 0.86 0.46
0.05 0.05 0.05 0.01
14.30 13.40 14.30 14.00
0.72 0.70 0.59 0.67
2.96 2.81 2.77 2.85
2.68 2.64 2.70 2.76
8.00 7.96 7.29 8.05
1.44 1.58 1.51 1.51
25.00 24.20 23.40 23.50
1.53 1.71 1.70 1.71
2.05 2.42 2.32 2.21
a
EDS, energy dispersive spectroscopy.
Fig. 1. X-ray diffractometry (XRD) results of the microstructure materials after heat treatments at different temperatures for 2 h.
488
T.W. Cheng et al. / Waste Management 22 (2002) 485–490
Fig. 1 also shows that the Bragg peak intensities were low after heat treatment for 2 h at 1150 C. This is presumably resulted from the relatively small driving force for the phase transformation. At this temperature, the atomic diffusion rate was high whereas the amounts of nucleation and crystallization were small. When treated at low temperatures such as 1050 or 950 C, the driving force for the phase transformation increased because more nuclei were obtained. When treated at 850 C, the driving force increased extensively while the atomic diffusion rate was low, resulting in formation of fewer new phases. 3.4. SEM microstructural examinations SEM microstructural examinations of microstructure materials after heat treatments at different temperatures were performed and the results are shown in Figs. 2–5.
Based on these micrographs, it can be seen that the microstructure materials were not thoroughly homogeneous. In general, the crystal size decreased with decreasing heat-treatment temperature. The average crystal size is around 0.2 mm (Figs. 4 and 5). These results are in a good agreement with XRD results. Generally, the crystallization process at a given temperature is controlled by nucleation and growth rates. Since both nucleation and growth rates vary oppositely with the temperature, it is expected that those two curves for nucleation and growth rates will overlap in a certain temperature range. Accordingly, in this study optimal temperatures for the heat treatment were identified to achieve better microstructure. When the microstructure materials were heat treated at 1150 or 1050 C, the crystal growth rate was improved whereas the crystallization was far from optimization due to limited nucleation rates. On the other hand, the heat treatment at low temperatures such as 950 or 850 C yields more
Fig. 2. Scanning electron microscopic (SEM) micrograph of microstructure material sample heat treated for 2 h at 1150 .
Fig. 4. Scanning electron microscopic (SEM) micrograph of microstructure material sample heat treated for 2 h at 950 C.
Fig. 3. Scanning electron microscopic (SEM) micrograph of microstructure material sample heat treated for 2 h at 1050 C.
Fig. 5. Scanning electron microscopic (SEM) micrograph of microstructure material sample heat treated for 2 h at 850 C.
T.W. Cheng et al. / Waste Management 22 (2002) 485–490
nuclei as well as an increased nucleation rate, thus producing a better and refined crystallization in materials microstructure (Figs. 4 and 5). Therefore, an improved, refined gehlenite microstructure material could be attained after the heat treatment at 850 or 950 C by using thermal plasma technology. 3.5. Physical and mechanical properties Various physical and mechanical properties of microstructure materials obtained from plasma vitrification with post-heat treatment at different temperatures are listed in Table 4. Densities of the microstructure materials were in the range of 2.9–3.0 g/cm3. Because these values are comparable with that of melilite (2.95 to 3.0 g/cm3) [12], the melilite as a major crystalline phase in the microstructure materials is then confirmed, and consistent with XRD results. The hardness properties were measured for feasibility evaluation of the materials for a given purpose. Hardness results shown in Table 4 reveal that the microstructure materials had Knoop hardness of 4.5–4.8 GPa and Mohs’ scale of 6.5–7. Thermal expansion is also one of important properties. As listed in Table 4, relatively low thermal-expansion coefficients suggest the microstructure materials obtained are suitable for the high-temperature applications. Furthermore, preferred evaluation results of other physical properties (water adsorption, porosity, and machineability) in Table 4 suggest the microstructure materials were fairly dense and thus may be useful for construction applications. It is interesting to note that many properties such as hardness, porosity, and water adsorption are improved with decreasing the heattreatment temperature. The improved properties are presumably attributed to the refined structure achieved in the low-temperature treated sample.
4. Conclusions Treatment and characterizations of the incinerated ash by using thermal plasma technology have been carried out. After post-heat treatments at different temperatures, microstructure materials with good quality Table 4 Various properties of the microstructure materials Properties
850 C
950 C
1050 C
1150 C
Density (g/cm3) Mohs’ hardness Knoop hardness (GPa) Porosity (%) Water absorption (%) Thermal expansion coefficient (10 6 mm C; 25–450 C) Machinability
2.99 7.0 4.84 0.83 0.28 9.85
2.96 7.0 4.70 1.81 0.61 8.21
2.94 6.5 4.62 2.68 0.91 8.68
2.93 6.5 4.57 2.31 0.79 8.41
Good
Good
Good
Good
489
are obtained. After plasma vitrification and one-step post-heat treatment, large volume and weight reduction ratios (6.4 and 1.7, respectively) are achieved. Heavy metals originated in the incinerated ash are confined in the treated silicate framework, hence yielding very low leachability results. Therefore, thermal plasma technology can be an effective method to treat incinerated ashes. After the high temperature melting and heat treatment process, microstructure materials were formed and consisted of predominantly a melilite group solid solution of gehlenite and a˚kermanite. It was found that the crystallization was progressively improved as the post-heat treatment temperature was decreased. The average grain size of the microstructure materials was 0.2 mm at the heat treatment temperature of 850– 950 C. Microstructure materials with improved physical and mechanical properties were thus obtained after being heat treated at low temperatures. Microstructure materials with preferred properties therefore have the potential to serve as a viable alternative for construction applications.
Acknowledgements The National Science Council of Republic of China supported this study under contract NSC-89-2211-E027-003, which was gratefully acknowledged.
References [1] Ling YC, Hou PCC. A Taiwanese study of 2,3,7,8-substituted PCDD/DFs and coplanar PCBs in fly ashes from incinerators. Journal of Hazardous Materials 1998;58:83–91. [2] Chu JP, Hwang IJ, Tzeng CC, Kuo YY, Yu YJ. Characterization of vitrified slag from mixed medical waste surrogates treated by a thermal plasma system. Journal of Hazardous Materials 1998;58: 172–92. [3] Romero M, Rinco´n J Ma. Preparation and properties of high iron oxide content glasses obtained from industrial wastes. Journal of the European Ceramic Society 1998;18:153–60. [4] Boccaccini AR, Bu¨cker J, Bossert J. Glass and glass-ceramaics from coal fly-ash and waste glass. Tile & Brick 1996;12(6):515–8. [5] Barbieri L, Manfredini T, Queralt I, Rinco´n J Ma, Romero M. Vitrification of fly ash from thermal power station. Glass Technology 1997;38(5):165–70. [6] Barbieri L, Lancellotti I, Manfredini T, Queralt I, Rinco´n J Ma, Romero M. Design, obtainment and properties of glasses and glass-ceramics from coal fly ash. Fuel 1999;78:271–6. [7] Rawlings RD. Production and properties of silceram glass-ceramic. Glass-ceramic Materials: Fundamentals and Applications. Modena 1997:115–33. [8] O¨vecog˘lu ML. Microstructural characterization and physical properties of a slag-based glass-ceramic crystallized at 950 and 11100 C. Journal of the European Ceramic Society 1998;18:161– 8. [9] Marabini AM, Plescia P, Maccari D, Burragato F, Pelino M. New materials from industrial and mining wastes: glass-ceramics and glass- and rock-wool fibre. International Journal of Mineral Processing 1998;53:121–34.
490
T.W. Cheng et al. / Waste Management 22 (2002) 485–490
[10] Boccaccini AR, Kopf M, Stumpfe W. Glass-ceramics from filter dust from waste incinerators. Ceramics International 1995;21: 231–5. [11] Romero M, Rawlings RD, Rinco´n JMa. Development of a new glass-ceramic by means of controlled vitrification and crystal-
lisation of inorganic wastes from urban incineration. Journal of the European Ceramic Society 1999;19:2049–58. [12] Deer WA, Howie RA, Zussman J. An introduction to the rock-forming minerals. Longman Scientific and Technical, 1992.