| Summary: | 博士 === 國立臺北科技大學 === 工程學院工程科技博士班 === 105 === In 1991, Taiwan EPA promulgated the "Management Methods for Reuse of MSWI Bottom Ash", establishing bottom ash as a reusable resource to be recycled. However, the promotion of this policy did not go smoothly, and bottom ash has not been well received by the public due to its pollution problems and quality, not to mention the fly ash, which has even higher pollutant content. This study investigates the effects of various pretreatments in reducing heavy metals and PCDD/Fs in MSWI ashes by analyzing the characteristics of heavy metals and dioxin, with an aim to reducing concerns about reutilization and serving as a reference for practical application.
According to the results of this study, the heavy metals in bottom ashes with the highest contents are Cu and Pb, and the highest TCLP-extractable concentration is found in Cu. In the reaction ash, the TCLP-extractable concentration of Pb is much higher than the standard level. The leaching of Cu and Pb is affected by the pH of the eluent, and the heavy metal bonding form may transform due to environmental conversion causing the substances to release when bottom ash is reused outdoors. The dioxin content and toxic equivalent concentration in fly ash were higher than that in bottom ash. In both fly ash and bottom ash, the OCDD had the highest contribution of the total PCDD/Fs content; based on I-TEQ, the highest contribution is 2,3,4,7,8-PeCDF. It is concluded that the PCDD/Fs characteristic index can be defined as: DCI = [(2,3,4,7,8-PeCDF content, ng/g) / (Total PCDD/Fs content, ng I-TEQ/g)]. In the case of this study, the DCI is 0.875 ± 7.6% for fly ash, and 0.589 ± 13.7% for bottom ash. It can be applied to characterize the fly ash discharged from the MSWI with the APCD assembly of SD, AC and BF and bottom ash discharged from MSWI with mass burning.In microwave-assisted acid degradation treatment (MDT) for reaction ash with microwave temperature set at 433K and treatment time at 180 minutes, the content of PCDD/Fs can be reduced from 35.221 ng/g.dw to 0.345 ng/g.dw. This work took the experimental data for correlation, and obtained the correlation equation between reduction efficiency and treatment time and temperature with a square correlation of 0.988 as follows: η (%) = 1- exp [-0.1875 × t × exp (-693.855/T) ]. Results of the laboratory and outdoor long-term leaching test indicate that phosphate can effectively reduce leaching of heavy metals in bottom ash such as Pb, Cu and Zn. Sodium silicate also has certain effect in stabilizing heavy metals, but there are some shortcomings in its application. Although washing bottom ash does not produce significant amount of heavy metal dissolution, it can reduce the potential of heavy metal dissolution. The Pb of fly ash was significantly dissolved by washing process.
Based on the results of this study, it is recommended that bottom ash be pretreated for reutilization by washing. The washing process has the advantages of dechlorination, deodorization and reducing the leaching potential of heavy metals. Through the designed process unburned matters and metals can once again be screened out, thus enhancing product quality. After washing, phosphate should be added to the bottom ash to enhance heavy metal stability. For bottom ash, the feasible pretreatments for reutilization are washing and sintering. The fly ash after washing is mixed with silica material and then sintered into aggregate. The fly ash with high concentration of pollutants from sintered process is degraded by microwave-assisted acid degradation treatment, and then returned to the sintering system as raw material. The chemical properties of the sintered aggregate are safe and reusable.
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