Article pubs.acs.org/IECR
Solidification of MSWI Ash at Low Temperature of 100 °C Chengchong Shan, Zhenzi Jing,* Li Pu, and Xiaohui Pan School of Materials Science and Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, China ABSTRACT: Municipal solid waste incineration (MSWI) bottom ash could be solidified with fly ash addition at 100 °C, and the flexural strength of solidified specimens reached almost 20 MPa. The strength development was found to be due mainly to C− S−H gel formation. Leaching tests were conducted to determine the amount of heavy metals dissolved from the solidified specimens, and the results showed that, under the hydrothermal conditions of this study, the leaching of heavy metals was very low. As such, it is possible to solidify MSWI ash by 100% (80% bottom ash + 20% fly ash) in a continuous production process with a low cost.
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the MSWI fly ash can be used as a calcium-rich material to solidify the bottom ash. The hardening mechanism and leaching properties were also studied to test the strength and security of solidified specimens. This study is expected to provide fundamental information for recycling MSWI ashes on a large scale.
INTRODUCTION Incineration is an efficient method for the treatment of municipal solid wastes, achieving a 90% volume reduction of wastes, the destruction of pathogenic agents, and the possible recovery of energy.1 Waste incineration plants have been built for incinerating municipal solid wastes in some large cities of China, producing a huge number of municipal solid waste incineration (MSWI) ashes (bottom ash and fly ash). In Shanghai, for example, about 20000 tons of fly ash is produced annually, and the amount is still growing quickly.2 Landfilling is now a common practice for the disposal of MSWI ashes. Road construction material is a major application for recycling MSWI bottom ash. Melting, chemical stabilization, cement solidification, and extraction for MSWI fly ash recycling have also been used. However, care needs to be taken to protect the environment because of the sizable quantities of heavy metals in MSWI, which can dissolve in rainfall and infiltrate into the underlying soil.3,4 In addition, the amount treated is only a small part of MSWI ashes produced to date.5 Therefore, finding a safe mass treatment/use for MSWI ashes is urgently needed. Recently, a hydrothermal technique has been applied to treat MSWI ashes in which the MSWI ashes could be solidified hydrothermally with tobermorite or calcium silicate hydrate (C−S−H) gel formation at low curing temperature (150−250 °C),6−10 and to form tobermorite or C−S−H gel, calcium-rich materials such as cement, slaked lime, and slag have to be added. Although the curing temperature in the hydrothermal process is quite low, special equipment consisting of an autoclave resistant to high pressure for the solidification is required, which not only raises the cost of MSWI ash recycling but also is unfavorable for continuous production processing of MSWI ash utilization. An autoclave will not be needed, however, if the hydrothermal process can be conducted at 100 °C, because the saturated steam pressure is equal to atmospheric pressure. The solidification process, therefore, could be conducted in a vessel made of bricks, such as a kiln in ceramics production, which is very helpful for realizing a continuous process. To the best of our knowledge, research dealing with the solidification of MSWI ashes at 100 °C is not easy to find. The objective of this work was to investigate whether MSWI ashes could be solidified at 100 °C and whether © 2012 American Chemical Society
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EXPERIMENTAL SECTION Materials and Preparation. The MSWI ashes used were obtained from an incineration plant in Shanghai, China. The MSWI fly ash was used as a raw material directly, whereas the bottom ash was ground with a ball mill to pass 150 μm. The chemical compositions and particle size distributions of the bottom ash and fly ash were determined by X-ray fluorescence (XRF) (model RIX3100, Rigaku) and laser particle-size analysis (LPSA) (model X100, Microtrac), respectively. Solidification. Ground bottom ash mixed with fly ash at different ratios was used as starting materials. The starting materials were first mixed with 15 mass % distilled water in a mortar by hand, and then the mixture was compacted by uniaxial pressing in a rectangular-shaped mold with a compaction pressure of 30 MPa. The demolded green specimens were subsequently solidified under saturated steam pressure at 100 °C for up to 96 h. After solidifying, all of the solidified specimens were dried at 80 °C for 24 h before testing. Analysis. The dried rectangular-shaped specimens (40 mm length × 15 mm width × 8 mm height) were used to determine the three-point flexural strength with a universal testing machine (model XQ160-A). The strengths presented here are averages from three measurements obtained for three different specimens. The crushed specimens were then investigated using several analytical techniques: for crystalline phase analysis and phase qualitative identification, X-ray diffraction (XRD) (model D/max 2550 VB3+/PC) and energy dispersive X-ray spectroscopy (EDS) (model Genesis X4m); for microscopic morphology, scanning electron microscopy (SEM) (model Quanta 200 FEG); for microstructure, Fourier transform Received: Revised: Accepted: Published: 9540
December 26, 2011 June 30, 2012 July 2, 2012 July 2, 2012 dx.doi.org/10.1021/ie203040d | Ind. Eng. Chem. Res. 2012, 51, 9540−9545
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infrared (FTIR) spectroscopy (model EQUINOXSS/HYPERION200); for porosity, mercury intrusion porosimetry (MIP) (model Poromaster GT-60, Quantachrome); and for heavy metals dissolved from the solidified specimens, inductively coupled plasma-mass spectrometry (ICP-MS) (model Agilent 7700) and inductively coupled plasma optical emission spectrometry (ICP-OES) (model Optima 2100DV). The leaching tests were conducted in accordance with the identification standards for hazardous wastes part 4 of China, identification of extraction toxicity (GB5085.3-2007), and by the standard method for solid waste-extraction procedure for leaching toxicity, horizontal vibration method (HJ557-2010).
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RESULTS AND DISCUSSION Characterization. The chemical compositions and particle size distributions of the bottom ash and fly ash are presented in Table 1 and Figure 1, respectively. It should be noted that, in Table 1. Compositions of Fly Ash and Bottom Ash CaO SiO2 Cl Na2O MgO Al2O3 P2O5 SO3 K2O TiO2 MnO2 Fe2O3 CuO ZnO SrO PbO Cr2O3 SnO2
fly ash (%)
bottom ash (%)
46.8 8.79 20.5 4.38 1.57 2.59 1.2 5.96 4.93 0.46 0.04 0.92 0.08 0.59 0.04 0.25 − −
27.9 35.5 1.24 2.69 2.61 6.73 4.74 6.32 1.72 0.78 0.08 3.73 0.09 0.37 0.07 0.12 0.02 0.13
Figure 1. Particle size distributions of (a) fly ash and (b) bottom ash.
effect of fly ash addition on the flexural strength of specimens solidified at 100 °C for 48 h was investigated first. The flexural strength shown in Figure 3, as expected, initially increased with increasing fly ash content, reached its highest value (∼19 MPa) at 20 mass %, and afterward started to decline, suggesting that the MSWI bottom ash could be solidified with MSWI fly ash addition. The phase evolution for the above specimens was investigated by XRD analysis (Figure 4). The peak intensity of quartz decreased gradually with fly ash addition, and at the same time, the peak corresponding to the C−S−H gel became wider and higher, indicating that the quartz reacted to form C− S−H gel, and the peak for C−S−H gel increased with increasing fly ash. However, the peak for C−S−H gel overlapped the peak for calcite. Because calcite is relatively steady under these conditions, the peak evolution might reflect mainly the evolution of the C−S−H gel. At 30 mass %, the peak intensity of the mineral corresponding to hydrocalumite [Ca8Al4(OH)24(CO3)Cl(H2O)9.6] became higher, and it increased afterward. Compared with the strength development (Figure 3), the strength enhancement was due to C−S−H gel formation. Based on work reported by Taylor,11 the C−S−H gel formed should be C−S−H (I), and our study showed that C−S−H (I) changes into 1.1-nm tobermorite with increasing temperature. However, more fly ash added will introduce more hydrocalumite and chloride (Cl). SEM (Figure 5) revealed the morphology of hydrocalumite formed at 40 mass %, which was similar to that reported by Perkins and Palmer12 and Zhang et
the bottom ash, the SiO2 content is higher than the CaO content, whereas in fly ash, the CaO content is much higher than the SiO2 content, and the Cl content is also much higher than that in bottom ash. Therefore, fly ash might have the potential to be used as a calcium-rich addition to solidify bottom ash. The mineralogical compositions of the raw fly ash and bottom ash measured by XRD are shown in Figure 2. The phases of fly ash correspond chiefly to calcium chloride hydroxide (CaClOH), sylvite (KCl), portlandite [Ca(OH)2], halite (NaCl), hydrocalumite [Ca 8 Al 4 (OH) 24 (CO 3 )Cl(H2O)9.6], and calcium sulfate (CaSO4), whereas the phases for bottom ash are mainly quartz (SiO2), calcite (CaCO3), anhydrite (CaSO 4 ), gehlenite (Ca 2 Al 2 SiO 7 ), anorthite (CaAl2Si2O8), and probably feldspar [R(AlSi3O8), where R = alkali metal]. Effect of Fly Ash Addition. Our previous work showed that, to enhance strength, slaked lime should be added to MSWI ash to form tobermorite or C−S−H gel.7 From Table1, the SiO2 content in bottom ash is higher than the CaO content, so lime should be added. As mentioned above, fly ash might be used as a calcium-rich additive and added to bottom ash in replacement of lime to form tobermorite or C−S−H gel. The 9541
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Figure 4. XRD patterns of solidified bodies with different fly ash contents. Hydrothermal processing conditions: curing time, 48 h; curing temperature, 100 °C.
Figure 5. SEM micrograph of a specimen solidified with 40% fly ash addition at 100 °C for 48 h.
chlorine introduction might result in the reduction of the strength of solidified bodies. The evolution of the reaction products was also characterized by FTIR analysis. As can be seen in Figure 6, the spectra of the solidified specimens exhibited peaks at about 3450 and 1650
Figure 2. XRD patterns of (a) fly ash and (b) bottom ash.
al.13 Hexagonal platelets with a size of around 1 μm formed in the spaces between the MSWI ash particles. Such a large crystal (hydrocalumite) formed among the ash particles and more
Figure 6. FTIR patterns of solidified specimens with different fly ash contents. Hydrothermal processing conditions: curing time, 48 h; curing temperature, 100 °C.
Figure 3. Effect of fly ash addition on flexural strength. Hydrothermal processing conditions: curing temperature, 100 °C; curing time, 48 h. 9542
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cm−1 due to molecular water (O−H) and deformation (H−O− H), respectively, indicating the presence of some hydration water in the reaction products.14 The two bands at around 1400 and 875 cm−1 correspond to CO32− anions resulting from the presence of CaCO3.15 In hydrated Portland cements, the main bands corresponding to C−S−H gel appear at ∼970 cm−1 [Si− O stretching vibrations of Si(Q2) tetrahedra], 660−670 cm−1 (Si−O−Si bending vibration), and 450−500 cm−1.16,17 With addition of fly ash, the peak at 970 cm−1, attributed to the Si−O stretching vibration in C−S−H gel, tended to appear at 10% and increased with larger fly ash contents. Although the C−S− H gel peak overlapped with the calcite peak in the XRD pattern evolution (Figure 4), according to the FTIR results, the peak of C−S−H gel tended to form clearly from 10 mass % fly ash addition, showing that C−S−H gel formation resulted in the strength enhancement shown in Figure 3. Effect of Curing Time. To elucidate the hardening mechanism during the hydrothermal process, the effect of curing time on the strength of specimens solidified at 100 °C with 20 mass % fly ash addition was investigated (Figure 7). The flexural strength increased rapidly up to 24 h, then increased more gently, and finally declined from 72 h onward.
Figure 8. XRD patterns of solidified bodies at different curing times. Hydrothermal processing conditions: fly ash content, 20 mass %; curing temperature, 100 °C.
Figure 7. Effect of curing time on the flexural strength of solidified bodies. Hydrothermal processing conditions: fly ash content, 20 mass %; curing temperature, 100 °C.
Figure 9. FTIR patterns of solidified bodies cured for different times. Hydrothermal processing conditions: fly ash content, 20 mass %; curing temperature, 100 °C.
The evolution of the phases investigated by XRD analysis for the same specimens is shown in Figure 8. At 0 h (before hydrothermal processing), the phases were mainly calcite, anhydrite, gehlenite, hydrocalumite, anorthite, and quartz. After 3 h, the peak intensity of quartz decreased, and from 6 h, the peak corresponding to C−S−H gel became higher significantly, suggesting that a reaction from quartz to C−S−H gel has occurred. Comparison of the strength development (Figure 7) with the XRD results (Figure 8) shows that the fast flexural strength development was due to C−S−H gel formation. It should be noted that the hydrocalumite phase was distinct at 0 h; however, it tended to disappear after 24 h, suggesting that hydrocalumite was broken down. The disappearance of hydrocalumite might be another reason for the increase in strength. FTIR spectroscopy was also conducted to investigate the evolution of phases with different curing times (Figure 9). The peak at 970 cm−1 due to Si−O stretching vibrations corresponding to the C−S−H gel tended to appear from 6 h
and then became higher for longer curing times. According to the XRD results (Figure 8), from 6 h, the peak of C−S−H gel also became significantly higher, which showed that C−S−H gel formation did improve the strength development of the solidified specimens. A detailed investigation of the microstructure evolution with increasing curing time was also conducted by measuring the change in porosity (Figure 10). Before hydrothermal treatment (0 h), the pores had a broad distribution between 0.02 and 7 μm. SEM (Figure 11a) revealed the pore dimension, corresponding to the voids between particles within the green specimen. The pore size distribution at 72 h shifted and narrowed to ∼0.01−0.07 μm. The peak shift reflects the reaction progress within the matrix, which resulted in the change in pore dimension. SEM of the specimen cured at 72 h (Figure 11b) indicated the presence of newly formed fibroid crystals, and some voids were filled with the newly formed crystals. According to the EDS results (Figure 11d) and the 9543
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leaching tests were carried out to determine the concentration of heavy metals dissolved from specimens solidified with 20 mass % fly ash addition at 100 °C for 72 h. Although the concentrations of heavy metals dissolved from the bottom ash and fly ash (Table 2) and the contents of heavy metals for both Table 2. Leaching Test Results for Fly Ash, Bottom Ash, and Solidified Specimens (mg/L)
Figure 10. Evolution of pore size distributions with increasing curing times (100 °C).
component
fly ash
bottom ash
specimen
Pb Cr Ni Hg Se Ba Cd As Sr
9.06 4.72 0.0190
