microstructural and morphological studies of incineration
MICROSTRUCTURAL AND MORPHOLOGICAL STUDIES OF INCINERATION BOTTOM ASH: MINERAL FORMATION AND ASSOCIATED PROCESSES M. UESHIMA*, H. SAKANAKURA *, H. KUBOTA**, K. SHIGEIZUMI** * Center for Material Cycles and Waste Management Research, National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba 305-8506, Japan ** Environmental Research Department, Fujita Corporation, Kanagawa, Japan
SUMMARY: Microscopic studies of air-cooled, water-quenched and aged incineration bottom ash (IBA) were performed by observing intact and thin-sectioned samples with optical/polarization microscope and FE-SEM/EDS. Polarization images of a particle in air-cooled IBA showed euhedral-shaped inclusions in a glassy matrix. Fluffy, porous and flakey particles were commonly found in air-cooled IBA. Elements present in IBA were localized in particles that may develop from air-cooling processes and sometimes form microphenocrysts. Water-wetting tests in air and N2 were conducted on IBA and afterwards analyzed by FE-SEM. IBA wetted in air showed calcite crystals covering the surface, whereas no calcite was found in IBA wetted in N2. This may indicate that CO2 present in air dissolved in water and reacted with Ca on IBA surface to form calcite. Water-quenched IBA and flux water column test performed for 50 days suggested Cl immobilization as secondary precipitates such as hydrocalumite or Friedel’s salt on the surface of IBA particles. Deeply-weathered IBA also showed secondary precipitates where each element was significantly localized and combined with other elements. This methodology would help characterize precipitates in IBA and potentially estimate the immobilization ratio of toxic elements inside these precipitates.
1. INTRODUCTION The assessment of chemical stability of toxic elements present in incineration bottom ash (IBA) is important because heavy metals and other toxic elements in IBA such as Cr, Pb, and Cl are condensed during incineration. These toxic elements could be released with leachates from landfills. Since the minerals and the chemical properties of IBA are crucial factors to determine the stability of toxic elements, we evaluated the microstructures of IBA by stereo microscopy, optical microscopy with several modes (transmitted and reflected lights and polarization filters), and field-emission scanning electron microscopy (FE-SEM) equipped with energy dispersive Xray spectrometer (EDS) techniques.
Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
2. MATERIALS AND METHODS 2.1 Incineration Bottom Ash (IBA) Municipal solid wastes are usually incinerated at temperatures around 800 oC or above. As a result, incineration bottom ash (IBA) and incineration fly ash (IFA) are produced. The IBA is rapidly cooled down in air to 200 oC approximately, and then dropped in a water pool to quench to room temperature. To understand the mineral formations in IBA during cooling processes, two types of IBA were collected from three incineration facilities in Japan. Air-cooled IBA was collected right after the incineration –it was cooled down to room temperature in air. In addition, water-quenched IBA was collected from the water pool and then dried in air. Besides, IBA aged in a yard outside an incineration facility for approximately 2 years was sampled and analyzed in order to understand the weathering process. 2.2 Microstructural/Morphological Characterization To characterize the microstructures and the morphologies of IBA in terms of cores formed during air-cooling or originally present minerals before incineration such as quartz and feldspars, and to identify secondary precipitates formed after quenching with water or during aging, the following preparations and analyses were conducted: § Embedding and Thin Sectioning § Optical/Polarization Microscopy § FE-SEM with Energy Dispersive X-ray Spectroscopy 2.2.1 Embedding and Thin Sectioning IBA particles were embedded for microscopic observations. IBA particles were immersed in acetone to dehydrate and to degas the surface. The samples were finally embedded in a lowviscosity epoxy resin (Spurr’s resin) to obtain a medium hardness (Spurr, 1969) and polymerized at 70 oC for 8 hours. Thin sections (ca. 30 µm in thickness) were prepared for microscopic observations or analyses. 2.2.2 Optical/Polarization Microscopy Observations of the thin-sectioned IBA were performed with an Olympus BX 51 microscope equipped with an Olympus Polarizer SP500F, in different modes (PPL, XPL, and RL), in order to understand the mineralogy of the IBA. 2.2.3 FE-SEM/EDS Observations and Analyses Microstructures/morphopogies were visualized by FE-SEM/EDS (JEOL JSM7800F/ Oxford Instruments AZtec 3.1) with an acceleration voltage of 15 kV. Intact minerals formed in aircooled IBA and secondary minerals on the surface of water-quenched and aged IBA were identified. Stabilities of the elements found on these particles were also discussed.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
3. RESULTS AND DISCUSSION 3.1 Air-cooled IBA 3.1.1 Optical microscope observations IBA was first observed by optical microscope. It was assumed that IBA did not undergo any chemical reaction such as mixture reactions with CO2. In other words, IBA was let to cool down at ambient conditions that did not favor oxidation or hydration. Figure 1 shows images of polarized microscopy of air-cooled IBA. These particles were cooled down to room temperature in air. During the cooling process in air, IBA gradually crystalized (Figure 1a), sometimes as euhedral-shaped inclusions (yellow arrows in Figure 1b).
Figure 1. Polarized microscopic images of IBA: (a) Crystalline particles and (b) euhedral inclusions (yellow arrows).
3.1.2 FE-SEM/EDS FE-SEM analysis was done for air-cooled IBA (Figure 2). Fluffy (2a), porous (2b), and flakey (2c) particles were identified. All of them contained Ca and O. The flakey particles also contained small amounts of Cl. Minerals that were originally present in wastes such as quartz and feldspars were also observed in IBA.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Figure 2. FE-SEM images of air-cooled IBA. Various morphologies were found: (a) fluffy, (b) porous and (c) flakey.
EDS elemental mapping was done for air-cooled IBA (Figure 3). Color contrasts represent relative concentrations of the elements. Figure 3 shows the localization of elements present in IBA. Even though in one aggregate, K and Cl were present together and Ti and Ca appear to coexist, Mg and O were found in both aggregates. Calcium was present in most of the particles. In air-cooled IBA, as shown in Figure 4, flakey aggregates were mainly composed of Ca, with small amounts of O and Cl.
Figure 3. FE-SEM images/EDS elemental mapping images of a porous aggregate in air-cooled IBA. Ca and Ti were found together in the same area, whereas K and Cl were found in another area together. Mg and O covered both areas.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Figure 4. FE-SEM image and EDS spectra of flaky aggregates in dry IBA. These aggregates were mainly composed of Ca and trace amount of Cl.
Air-cooled IBA also showed a unique shape inside a particle. Figure 5 shows cross-sectioned FE-SEM/EDS images and elemental mapping for air-cooled IBA. The bright part in the backscattered (BS) image only contains Fe and O whereas other elements such as Si, Al, Ca, Na, and K are present in the dark part of the BS. This phenomenon occurs due to the solidification properties of each compound. For instance, the solidification point of FeOx ranges between 1377 and 1565 oC whereas the temperature for feldspars to become a solid solution ranges between 650 and 900 oC (Klein and Philpotts 2017). When alumino-silicates (i.e. feldspar, etc.) reach these temperature ranges, elements such as Si, Al, Ca, Na and K start to homogenize. In this case, Fe was excluded from the alumino-silicates, cooled down and solidified independently. However, another particle showed a different behavior. Euhedral microphenocrysts containing Ca, Si and O were formed in the particle, whereas the surrounding matrix consisted of Na, Si and O (Figure 6). As shown in Figure 6, there was no carbon in the microphenocrysts, which indicates that microphenocrysts were not carbonate species.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Figure 5. FE-SEM images and the EDS elemental mapping images of the thin-sectioned dried IBA. Iron was detected in bright parts in the FE-SEM electron-backscattered image (BS) and other cations were found in the dark parts. Oxygen was uniformly present in both bright and dark parts.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Figure 6. FE-SEM images and EDS elemental mapping images of the thin-sectioned dry IBA. Calcium was found in bright parts in FE-SEM electron-backscattered image (BS) and Na was present in places were no calcium was found. Silicon and oxygen were uniformly distributed. Carbon was not clearly identified.
3.2 IBA with Water Treatment and Aging 3.2.1 Water-wetted IBA Wetting experiment was done for air-cooled IBA to visually follow and understand the reaction. As shown in Figure 7, intact air-cooled surface was porous (Figure 7a). The porous surface of IBA changed after wetting it with water in air. As a result, euhedal crystals (calcite) precipitated on the surface (Figure 7b). However, no calcite mineralization was identified on IBA surface after wetting it with water in N2 (Figure 7c). Thus, ambient conditions may impact and define the precipitates that crystallize on IBA surface even in the same moisture/wetting condition. In this particular case, CO2 was present in the air and, therefore, the HCO3 dissolved
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
in water reacted with Ca on IBA surface.
Figure 7. FE-SEM images of air-cooled IBA wetted with water: (a) before adding water, (b) after adding water in air (euhedral secondary precipitates on IBA surface) and (c) after adding water in N2.
3.2.2 IBA aged for 50 days Aging test of IBA against flux water was conducted in a column for 50 days. Before the aging test, a porous spherical particle with precipitates on the surface containing Si, Al, O, Cl and Ca was identified (Figure 8a). This image, showing a thin-sectioned water-quenched IBA, suggests that an air-cooled spherical particle releases several elements such as Ca, Al, and O, and probably Si, after the quenching in a water pool. These elements may capture Cl and precipitate on the surface of the spherical particle. Figure 8b shows another water-quenched particle with precipitates after a 50-day aging. The elemental contents were the same as Figure 8a, but the elemental distribution was slightly different. The distribution of Ca and O were similar between (a) and (b) in Figure 8. The release of Si and Al were identified by comparing their distributions in (a) and (b). Regarding Cl, there was no significant distribution in the core of the particles in both (a) and (b), but in the precipitates (i.e. surrounding the cores). It appears that Cl came from outside the particles, and was captured as aging took place, according to the color contrast for Cl in Figure 8b. Recent publications regarding mineral speciation of IBA after quenching with water have indicated that Friedel’s salt (Ca2Al(OH)6Cl·2H2O) and/or hydrocalumite (Ca2Al(OH)6[Cl1-x(OH)x]·3H2O) (Wei et al. 2011, Inakaew et al. 2016, Saffarzadeh et al. 2016) were formed. The result of the 50-day aging test did not show significant precipitation of Si. However, Ca, Al, Cl and O precipitations correspond to X-ray diffraction results reported in previous studies.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Figure 8. FE-SEM images and EDS elemental mapping images of IBA obtained from a quenching water pool in a municipal solid waste incineration plant: a particle (a) right after quenching, (b) after aging with flow water and air.
3.3 Deeply-Weathered IBA We observed and studied the aging process of IBA in a yard. Figure 9 shows the images obtained by FE-SEM/EDS. The core (upper part of every image in Figure 9) contained Na, Si, K, O and Al, which suggests the presence of alkali feldspar. Besides, the precipitates in the lower part of the core contained Na, Si, Cl and O. Looking at the precipitates, Na, Si and Ca appear to be present in the same areas, whereas Al existed elsewhere; and O covered all the precipitate (Figure 9). We have neither clarified Cl distribution in the precipitates nor identified the microstructures of these precipitates yet. However, further investigation using this methodology would help identify the precipitates and potentially estimate the stability of toxic elements inside these precipitates.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Figure 9. FE-SEM images and EDS elemental mapping images of IBA obtained from a municipal solid waste incineration plant. The IBA was weathered for 2 years on the surface of the yard outside the plant. 4. CONCLUSIONS Microscopic studies of air-cooled, water-quenched and aged IBA were performed by observing intact and thin-sectioned samples with an optical/polarization microscope and an FESEM/EDS. Polarization images of a particle in air-cooled IBA showed euhedral-shaped inclusions in a glassy matrix. Fluffy, porous and flakey particles were commonly found in aircooled IBA. Elements present in IBA were localized in a particle after air-cooling processes and optical analysis results showed the formation of microphenocrysts. Water-wetting tests in air and N2 were visualized by FE-SEM. IBA wetted in air showed calcite crystals covering the surface, whereas no calcite was found in the surface of IBA wetted in N2. This suggests that CO2 in air dissolved in water and reacted with Ca on IBA surface. Water-quenched IBA and flux water column test for 50 days suggested Cl immobilization as hydrocalumite or Friedel’s salt, secondary precipitates on the surface of IBA particles. Deeply-weathered IBA also showed secondary precipitates where each element was significantly localized and combined with other elements. This morphological characterization may constitute an important tool to evaluate the stability of IBA. Such mineral formations in the cooling processes may represent the key to determine the mobility of toxic elements in landfill leachates. Therefore, a deep understanding of minerals, their properties and stabilities will provide an accurate way to design, construct and monitor landfill systems.
ACKNOWLEDGEMENTS The authors are grateful to those who provided IBA samples. The authors also gratefully acknowledge the comments and suggestions of reviewers that improved the quality of this manuscript.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
REFERENCES Inkaew K., Saffarazadeh A. and Shimaoka T. (2016) Modeling the formation of the quench product in municipal solid waste incineration (MSWI) bottom ash, Waste Management, vol52, 159-168 Klein C. and Philopotts A.R. (2017) Earth materials: Introduction to mineralogy and petrology, Cambridge University PressCambridge, 159-195. Saffarazadeh A., Arumugam N. and Shimaoka T. (2016) Aluminum and aluminum alloys in municipal solid waste incineration (MSWI) bottom ash: A potential source for the production of hydrogen gas, Intl. J. Hydrogen Energy. vol41, 820-831. Spurr A.R. (1969) A low-viscosity epoxy resin embedding medium for electron microscopy, J. Ultrastruct Res., vol26, 31-43. Wei Y., Shimaoka T., Saffarazadeh A. and Takahashi F. (2011) Mineralogical characterization of municipal solid waste incineration bottom ash with an emphasis on heavy metal-bearing phases, J. Hazard. Mater., vol187, 534-543.
SUMMARY: Microscopic studies of air-cooled, water-quenched and aged incineration bottom ash (IBA) were performed by observing intact and thin-sectioned samples with optical/polarization microscope and FE-SEM/EDS. Polarization images of a particle in air-cooled IBA showed euhedral-shaped inclusions in a glassy matrix. Fluffy, porous and flakey particles were commonly found in air-cooled IBA. Elements present in IBA were localized in particles that may develop from air-cooling processes and sometimes form microphenocrysts. Water-wetting tests in air and N2 were conducted on IBA and afterwards analyzed by FE-SEM. IBA wetted in air showed calcite crystals covering the surface, whereas no calcite was found in IBA wetted in N2. This may indicate that CO2 present in air dissolved in water and reacted with Ca on IBA surface to form calcite. Water-quenched IBA and flux water column test performed for 50 days suggested Cl immobilization as secondary precipitates such as hydrocalumite or Friedel’s salt on the surface of IBA particles. Deeply-weathered IBA also showed secondary precipitates where each element was significantly localized and combined with other elements. This methodology would help characterize precipitates in IBA and potentially estimate the immobilization ratio of toxic elements inside these precipitates.
1. INTRODUCTION The assessment of chemical stability of toxic elements present in incineration bottom ash (IBA) is important because heavy metals and other toxic elements in IBA such as Cr, Pb, and Cl are condensed during incineration. These toxic elements could be released with leachates from landfills. Since the minerals and the chemical properties of IBA are crucial factors to determine the stability of toxic elements, we evaluated the microstructures of IBA by stereo microscopy, optical microscopy with several modes (transmitted and reflected lights and polarization filters), and field-emission scanning electron microscopy (FE-SEM) equipped with energy dispersive Xray spectrometer (EDS) techniques.
Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
2. MATERIALS AND METHODS 2.1 Incineration Bottom Ash (IBA) Municipal solid wastes are usually incinerated at temperatures around 800 oC or above. As a result, incineration bottom ash (IBA) and incineration fly ash (IFA) are produced. The IBA is rapidly cooled down in air to 200 oC approximately, and then dropped in a water pool to quench to room temperature. To understand the mineral formations in IBA during cooling processes, two types of IBA were collected from three incineration facilities in Japan. Air-cooled IBA was collected right after the incineration –it was cooled down to room temperature in air. In addition, water-quenched IBA was collected from the water pool and then dried in air. Besides, IBA aged in a yard outside an incineration facility for approximately 2 years was sampled and analyzed in order to understand the weathering process. 2.2 Microstructural/Morphological Characterization To characterize the microstructures and the morphologies of IBA in terms of cores formed during air-cooling or originally present minerals before incineration such as quartz and feldspars, and to identify secondary precipitates formed after quenching with water or during aging, the following preparations and analyses were conducted: § Embedding and Thin Sectioning § Optical/Polarization Microscopy § FE-SEM with Energy Dispersive X-ray Spectroscopy 2.2.1 Embedding and Thin Sectioning IBA particles were embedded for microscopic observations. IBA particles were immersed in acetone to dehydrate and to degas the surface. The samples were finally embedded in a lowviscosity epoxy resin (Spurr’s resin) to obtain a medium hardness (Spurr, 1969) and polymerized at 70 oC for 8 hours. Thin sections (ca. 30 µm in thickness) were prepared for microscopic observations or analyses. 2.2.2 Optical/Polarization Microscopy Observations of the thin-sectioned IBA were performed with an Olympus BX 51 microscope equipped with an Olympus Polarizer SP500F, in different modes (PPL, XPL, and RL), in order to understand the mineralogy of the IBA. 2.2.3 FE-SEM/EDS Observations and Analyses Microstructures/morphopogies were visualized by FE-SEM/EDS (JEOL JSM7800F/ Oxford Instruments AZtec 3.1) with an acceleration voltage of 15 kV. Intact minerals formed in aircooled IBA and secondary minerals on the surface of water-quenched and aged IBA were identified. Stabilities of the elements found on these particles were also discussed.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
3. RESULTS AND DISCUSSION 3.1 Air-cooled IBA 3.1.1 Optical microscope observations IBA was first observed by optical microscope. It was assumed that IBA did not undergo any chemical reaction such as mixture reactions with CO2. In other words, IBA was let to cool down at ambient conditions that did not favor oxidation or hydration. Figure 1 shows images of polarized microscopy of air-cooled IBA. These particles were cooled down to room temperature in air. During the cooling process in air, IBA gradually crystalized (Figure 1a), sometimes as euhedral-shaped inclusions (yellow arrows in Figure 1b).
Figure 1. Polarized microscopic images of IBA: (a) Crystalline particles and (b) euhedral inclusions (yellow arrows).
3.1.2 FE-SEM/EDS FE-SEM analysis was done for air-cooled IBA (Figure 2). Fluffy (2a), porous (2b), and flakey (2c) particles were identified. All of them contained Ca and O. The flakey particles also contained small amounts of Cl. Minerals that were originally present in wastes such as quartz and feldspars were also observed in IBA.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Figure 2. FE-SEM images of air-cooled IBA. Various morphologies were found: (a) fluffy, (b) porous and (c) flakey.
EDS elemental mapping was done for air-cooled IBA (Figure 3). Color contrasts represent relative concentrations of the elements. Figure 3 shows the localization of elements present in IBA. Even though in one aggregate, K and Cl were present together and Ti and Ca appear to coexist, Mg and O were found in both aggregates. Calcium was present in most of the particles. In air-cooled IBA, as shown in Figure 4, flakey aggregates were mainly composed of Ca, with small amounts of O and Cl.
Figure 3. FE-SEM images/EDS elemental mapping images of a porous aggregate in air-cooled IBA. Ca and Ti were found together in the same area, whereas K and Cl were found in another area together. Mg and O covered both areas.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Figure 4. FE-SEM image and EDS spectra of flaky aggregates in dry IBA. These aggregates were mainly composed of Ca and trace amount of Cl.
Air-cooled IBA also showed a unique shape inside a particle. Figure 5 shows cross-sectioned FE-SEM/EDS images and elemental mapping for air-cooled IBA. The bright part in the backscattered (BS) image only contains Fe and O whereas other elements such as Si, Al, Ca, Na, and K are present in the dark part of the BS. This phenomenon occurs due to the solidification properties of each compound. For instance, the solidification point of FeOx ranges between 1377 and 1565 oC whereas the temperature for feldspars to become a solid solution ranges between 650 and 900 oC (Klein and Philpotts 2017). When alumino-silicates (i.e. feldspar, etc.) reach these temperature ranges, elements such as Si, Al, Ca, Na and K start to homogenize. In this case, Fe was excluded from the alumino-silicates, cooled down and solidified independently. However, another particle showed a different behavior. Euhedral microphenocrysts containing Ca, Si and O were formed in the particle, whereas the surrounding matrix consisted of Na, Si and O (Figure 6). As shown in Figure 6, there was no carbon in the microphenocrysts, which indicates that microphenocrysts were not carbonate species.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Figure 5. FE-SEM images and the EDS elemental mapping images of the thin-sectioned dried IBA. Iron was detected in bright parts in the FE-SEM electron-backscattered image (BS) and other cations were found in the dark parts. Oxygen was uniformly present in both bright and dark parts.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Figure 6. FE-SEM images and EDS elemental mapping images of the thin-sectioned dry IBA. Calcium was found in bright parts in FE-SEM electron-backscattered image (BS) and Na was present in places were no calcium was found. Silicon and oxygen were uniformly distributed. Carbon was not clearly identified.
3.2 IBA with Water Treatment and Aging 3.2.1 Water-wetted IBA Wetting experiment was done for air-cooled IBA to visually follow and understand the reaction. As shown in Figure 7, intact air-cooled surface was porous (Figure 7a). The porous surface of IBA changed after wetting it with water in air. As a result, euhedal crystals (calcite) precipitated on the surface (Figure 7b). However, no calcite mineralization was identified on IBA surface after wetting it with water in N2 (Figure 7c). Thus, ambient conditions may impact and define the precipitates that crystallize on IBA surface even in the same moisture/wetting condition. In this particular case, CO2 was present in the air and, therefore, the HCO3 dissolved
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
in water reacted with Ca on IBA surface.
Figure 7. FE-SEM images of air-cooled IBA wetted with water: (a) before adding water, (b) after adding water in air (euhedral secondary precipitates on IBA surface) and (c) after adding water in N2.
3.2.2 IBA aged for 50 days Aging test of IBA against flux water was conducted in a column for 50 days. Before the aging test, a porous spherical particle with precipitates on the surface containing Si, Al, O, Cl and Ca was identified (Figure 8a). This image, showing a thin-sectioned water-quenched IBA, suggests that an air-cooled spherical particle releases several elements such as Ca, Al, and O, and probably Si, after the quenching in a water pool. These elements may capture Cl and precipitate on the surface of the spherical particle. Figure 8b shows another water-quenched particle with precipitates after a 50-day aging. The elemental contents were the same as Figure 8a, but the elemental distribution was slightly different. The distribution of Ca and O were similar between (a) and (b) in Figure 8. The release of Si and Al were identified by comparing their distributions in (a) and (b). Regarding Cl, there was no significant distribution in the core of the particles in both (a) and (b), but in the precipitates (i.e. surrounding the cores). It appears that Cl came from outside the particles, and was captured as aging took place, according to the color contrast for Cl in Figure 8b. Recent publications regarding mineral speciation of IBA after quenching with water have indicated that Friedel’s salt (Ca2Al(OH)6Cl·2H2O) and/or hydrocalumite (Ca2Al(OH)6[Cl1-x(OH)x]·3H2O) (Wei et al. 2011, Inakaew et al. 2016, Saffarzadeh et al. 2016) were formed. The result of the 50-day aging test did not show significant precipitation of Si. However, Ca, Al, Cl and O precipitations correspond to X-ray diffraction results reported in previous studies.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Figure 8. FE-SEM images and EDS elemental mapping images of IBA obtained from a quenching water pool in a municipal solid waste incineration plant: a particle (a) right after quenching, (b) after aging with flow water and air.
3.3 Deeply-Weathered IBA We observed and studied the aging process of IBA in a yard. Figure 9 shows the images obtained by FE-SEM/EDS. The core (upper part of every image in Figure 9) contained Na, Si, K, O and Al, which suggests the presence of alkali feldspar. Besides, the precipitates in the lower part of the core contained Na, Si, Cl and O. Looking at the precipitates, Na, Si and Ca appear to be present in the same areas, whereas Al existed elsewhere; and O covered all the precipitate (Figure 9). We have neither clarified Cl distribution in the precipitates nor identified the microstructures of these precipitates yet. However, further investigation using this methodology would help identify the precipitates and potentially estimate the stability of toxic elements inside these precipitates.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Figure 9. FE-SEM images and EDS elemental mapping images of IBA obtained from a municipal solid waste incineration plant. The IBA was weathered for 2 years on the surface of the yard outside the plant. 4. CONCLUSIONS Microscopic studies of air-cooled, water-quenched and aged IBA were performed by observing intact and thin-sectioned samples with an optical/polarization microscope and an FESEM/EDS. Polarization images of a particle in air-cooled IBA showed euhedral-shaped inclusions in a glassy matrix. Fluffy, porous and flakey particles were commonly found in aircooled IBA. Elements present in IBA were localized in a particle after air-cooling processes and optical analysis results showed the formation of microphenocrysts. Water-wetting tests in air and N2 were visualized by FE-SEM. IBA wetted in air showed calcite crystals covering the surface, whereas no calcite was found in the surface of IBA wetted in N2. This suggests that CO2 in air dissolved in water and reacted with Ca on IBA surface. Water-quenched IBA and flux water column test for 50 days suggested Cl immobilization as hydrocalumite or Friedel’s salt, secondary precipitates on the surface of IBA particles. Deeply-weathered IBA also showed secondary precipitates where each element was significantly localized and combined with other elements. This morphological characterization may constitute an important tool to evaluate the stability of IBA. Such mineral formations in the cooling processes may represent the key to determine the mobility of toxic elements in landfill leachates. Therefore, a deep understanding of minerals, their properties and stabilities will provide an accurate way to design, construct and monitor landfill systems.
ACKNOWLEDGEMENTS The authors are grateful to those who provided IBA samples. The authors also gratefully acknowledge the comments and suggestions of reviewers that improved the quality of this manuscript.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
REFERENCES Inkaew K., Saffarazadeh A. and Shimaoka T. (2016) Modeling the formation of the quench product in municipal solid waste incineration (MSWI) bottom ash, Waste Management, vol52, 159-168 Klein C. and Philopotts A.R. (2017) Earth materials: Introduction to mineralogy and petrology, Cambridge University PressCambridge, 159-195. Saffarazadeh A., Arumugam N. and Shimaoka T. (2016) Aluminum and aluminum alloys in municipal solid waste incineration (MSWI) bottom ash: A potential source for the production of hydrogen gas, Intl. J. Hydrogen Energy. vol41, 820-831. Spurr A.R. (1969) A low-viscosity epoxy resin embedding medium for electron microscopy, J. Ultrastruct Res., vol26, 31-43. Wei Y., Shimaoka T., Saffarazadeh A. and Takahashi F. (2011) Mineralogical characterization of municipal solid waste incineration bottom ash with an emphasis on heavy metal-bearing phases, J. Hazard. Mater., vol187, 534-543.
