| 摘要: | 本研究利用水洗前處理與添加含矽氧物質調質方式來改善都市垃圾焚化飛灰燒結試體之重金屬揮發與溶出特性，並增加於材料上的應用性。實驗用水洗灰以固液比1：100比例將飛灰水洗半小時後，烘乾備用。添加劑則選用含矽氧之矽酸、高嶺土與下水污泥灰。 實驗結果顯示，重金屬的揮發與金屬存在形態和燒結效果有關。飛灰經水洗前處理後可去除灰中90%以上的Cl，40%Na、K以及20%左右Ca和S。經水洗去除氯鹽後，可避免重金屬形成易揮發之金屬氯化物，降低Cd、Pb於低溫時(800℃)的揮發性。高溫下（900-1000℃），受水洗程序將低熔點鹽類去除的影響，使試體不易產生液相包覆效果，導致Cd、Pb的揮發率反較原灰為高，但對Cu、Zn、Cr等不易揮發的金屬則影響不大。水洗灰在添加含矽氧物質調質後，可減少重金屬於高溫燒結時的揮發，並增加重金屬形成穩定礦物相的可能。但原灰添加矽氧後，受矽氧易與氯鹽競爭陽離子（Na、K、Ca）的影響，會釋放出大量自由氯，增加了試體中重金屬的揮發，尤其對重金屬Cu、Zn揮發的增加最為顯著。 在試體中重金屬相分布特性方面，實驗結果顯示重金屬溶出與其分布於碳酸鹽相與離子交換相的比例有關。飛灰經水洗後可大幅降低燒結體中重金屬Cd、Zn、Pb分布於可離子交換相與碳酸鹽相的比例，進而減少溶出的可能。經水洗後，重金屬Cr於600-800℃的燒結溫度下易與K結合成易溶性之K2CrO4，導致燒結體溶出增加。高溫下（900℃-1000℃）Cr可形成Ca3Cr2(SiO4)3、(Fe,Mg)(CrFe)2O4等不易溶出之物種，使其溶出濃度降至法規標準以下。水洗飛灰於添加矽氧物質後，可增加重金屬Cd、Zn和Cr與矽氧結合形成穩定矽酸鹽類礦物的機率，從而降低了TCLP的溶出濃度。 在燒結體材料特性方面，飛灰經水洗前處理後會導致試體抗壓強度與健性的下降，於添加矽氧物質後，可大幅地增強試體之抗壓強度與健性，以達更廣之應用性。 To reduce heavy metal evaporation during thermal treatment of MSWI fly ash, and to obtain a sintered ash with low heavy metal leachability and better mechanical properties, this study investigated the effects of a water washing pre-treatment and SiO2-containing additives (silicic acid, kaolinite and sludge ash) on the mobility of heavy metals (Cd, Zn, Cu and Cr) in the fly ash. The results indicate that the evaporation of heavy metals in fly ash depends on the volatility of heavy metals and/or their compounds, the presence of chlorides, low melting components, and silica oxides. A water washing process at a liquid to solid ratio of 1:100 removed approximately more than 90% chloride ions, 40% Na and K, and about 20% Ca and S from the fly ash. The washing process removed most of the soluble chlorides necessary for heavy metals to form volatile metallic chlorides, thus decreasing the evaporation of volatile Cd and Pb at 600-800℃. However, this process also removed the low melting point components which work to immobilize heavy metals during heating, thereby resulting in increasing evaporation of Cd and Pb againe at 800-1000℃. With respect to the medium to low volatile heavy metals such as Zn, Cu and Cr, no significant effects were noted on their evaporization caused by the washing process. The addition of Silica in washed ash reduced the volatilization of heavy metals during sintering process by forming inert mineral phases. On the other hand, the presence of silica in raw fly ash samples enhanced the ability of chlorides (NaCl, KCl, CaCl2) to release chlorine by forming silicates. The available Cl then attacked the heavy metal oxides to form metallic chlorides and increased heavy metal volatilization, especially for Cu and Zn. The results also indicates that the leaching of heavy metals were related to the fractions bound to carbonates and exchangeable cations. A washing step can significantly decrease the heavy metals located in the above fractions, thus decreasing the Cd, Pb, and Zn concentration in the TCLP leachate of the sintered ash. Heating the washed fly ash at 600-800℃ increased the Cr fraction bound to the readily exchangeable cations as the Cr was converted into soluble K2CrO4, whereas heating the washed ash at 800-1000℃resulted in the formation of Ca3Cr2(SiO4)3 and (Fe,Mg)(CrFe)2O4, thereby decreasing the Cr concentrations in the TCLP leachate. The SiO2-containing additives were also found to have enhanced the formation of insoluble silicates and thus the Cd, Zn, and Cr leaching concentration. The additives were also found to have improved the decreased compressive strength and soundness of the washed-ash monoliths caused by the washing process. |
