| 摘要: | 本研究利用酸萃取技術，探討都市垃圾焚化飛灰無害化及重金屬回收之可行性，第一階段研究以 1 N 硫酸與焚化飛灰進行混合，攪拌轉速調整為 50 rpm，並控制萃取溶液 pH 值(2、3、4 及 5)且保持恆定，以及設定萃取時間(30、60、90 及 120 分鐘)，再根據重金屬萃取之總量分析結果，評
估最佳萃取條件。第二階段研究 0.25 N 氫氧化鈉溶液控制 pH=10 之條件下，評估化學沉澱反應形成鉛、鋅及銅重金屬化合物之回收率及純度。研
究結果顯示，較低之 pH 值易使重金屬溶出，其中 pH=2 萃取 60 min 之條件下，鉛、鋅及銅萃取率分別為 10.83%、27.16%及 19.89%。根據XRD 物種鑑定及模式模擬之物種分析結果得知，在 pH 值小於 5 之條件下，鋅及銅化合物較容易被溶解，其中銅可能形成少量之氧氯化銅及硫化亞銅沉澱，鉛則主要以二氧化鉛存在。此外，硫酸根離子會與鉛形成硫酸鉛沉澱，因此，硫酸較不易萃取鉛金屬，其萃取率均較其他重金屬為低。進一步評估前述條件下，重金屬鉛、鋅及銅之回收成效，當 pH 值控制在 10時，鉛、
鋅及銅之回收率分別為 12.90%、31.12%及 25.59%，其中重金屬鋅回收純
度最佳，約為 19.98%。
根據毒性溶出試驗結果顯示，經 pH=2 萃取 60 min 後之焚化飛灰，鉛溶出濃度從 36.21 mg/L 降至 0.79 mg/L，其餘重金屬溶出濃度皆低於法規管制標準，可見本研究焚化飛灰經酸萃取後，已達無害化之目的。進一步根據序列萃取之相態分布特性結果顯示，焚化飛灰中易溶出之可交換態含量較高，酸萃取後之焚化飛灰相態，已轉換為較穩定之相態，其中 pH=2 萃取 60 min 之條件下，飛灰中重金屬相態已從可交換態及碳酸鹽結合態轉換為較穩定之鐵錳氧化態、有機物結合態及殘餘態。本研究嘗試以風險評估指標(RAC)，評估酸萃取後之飛灰在環境中之相對風險程度，結果顯示焚化飛灰可交換態含量高，鉛、鋅及銅之 RAC 風險指數分別為 27.42、87.55及 60.30，故屬於高風險，而經 pH=2 萃取 60 min 條件後，可使飛灰中鉛、鋅及銅金屬風險指數，分別降至 19.16、18.74 及 2.08，相對風險性已從高風險降至中低風險之特性。
整體而言，本研究已初步驗證酸萃取技術，可有效將焚化飛灰之重金
屬萃取與回收，且萃取後焚化飛灰之重金屬相態已轉換為較穩定之相態，
同時達到無害化與降低後續處理處置之環境風險。
;This research investigated the feasibility of non-hazardous and heavy metal recovery of municipal solid waste incineration (MSWI) fly ash by acid extraction. In the first stage of this research, 1 N sulfuric acid was mixed with fly ash and controlled the stirring speed at 50 rpm. Meanwhile the pH value adjusted ranged between 2 and 5, and operated the extraction time at 30, 60, 90, and 120 minutes.In the second stage, the recovery rate and purity of lead, zinc, and copper compounds formed by chemical precipitation with controlling pH=10 by adding 0.25 N sodium hydroxide solution. The results showed that the lower pH value is easy to dissolve heavy metals. In the case of pH=2 and the extraction time was 60 minutes, the extraction rates of lead, zinc, and copper were 10.83%, 27.16%,and 19.89%, respectively. According to the results of metal speciation identified by XRD and simulated by the model, zinc and copper compounds were more easily dissolved in the case of pH values less than 5. Copper may form copper oxychloride and copper sulfide precipitates, and lead was mainly presented as lead dioxide. In addition, sulfate ions would react with lead to form lead sulfate precipitation. Therefore, sulfuric acid was more difficult to extract lead, and its extraction rate was lower than other heavy metals. Further evaluation of the recovery efficiency of heavy metals lead, zinc, and copper under the above conditions, the recovery rates of lead, zinc, and copper were 12.90%, 31.12%,and 25.59%, respectively. The purity of zinc recovered from the extraction solution was approximately 19.98%.
According to the results of toxicity characteristics leaching procedure (TCLP), in the case of pH=2 and extraction time was 60 minutes, lead TCLP concentration decreased from 36.21 mg/L to 0.79 mg/L, and the leaching concentrations of other heavy metals were lower than the regulatory standards.Therefore, MSWI fly ash used in this research had achieved the purpose of non hazardous treatment after acid extraction process. The sequential extraction
further analyzed the phase characteristics of fly ash. The results showed that the content of exchangeable phase in the fly ash was relatively high. However, extracted fly ash′s metal phase distribution converted to a more stable phase
states after acid extraction. In the case of pH=2 and extraction time was 60 minutes, the exchangeable phase and the carbonate-bound phase in the fly ash were dissolved and converted to the more stable Fe/Mn oxide phase, organic
phase and residual phase. This research attempted to use the risk assessment code(RAC) to evaluate the relative environmental risk of fly ash after acid extraction.The analysis results showed that the RAC indices of lead, zinc, and copper were 27.42, 87.55, and 60.30. It implied that fly ash was considered a high-risk hazardous waste. In the case of pH=2 and extraction time was 60 minutes, the RAC indices of lead, zinc, and copper in fly ash decreased to 19.16, 18.74, and 2.08. That is, the extracted fly ash was presented with relative medium and low risk characteristics. |
