| dc.description.abstract | 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 nonhazardous 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 lowrisk characteristics. | en_US |