Development of Advanced Cleaning Technology for Hazardous Heavy Metals in Coal

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About the hazardous heavy metals in coal and coal ash, the development of measurement methods and the studies on the removal and leaching behaviors were carried out. The results obtained in this study are as follows.1. For the various elements in coal and coal fly ash, the measurement methods were established. The optimum conditions for the acid-decomposition by use of microwave processor were assessed. For volatile elements, such as Hg and F, the oxygen flask combustion method was also successful. The decomposition solutions obtained were analyzed, and the cold vapor AAS for Hg, hydride generation AAS for As and Se, ion-selective electrode for F, and ICP-AES for other elements were applied.2. The leaching behavior of various elements from coal to diluted acid solutions and chelating agent solutions was examined. Among heavy metals, Mn was easy to leach, while Cu, Pb, and Zn were also considerably leached. The leaching amount of element was almost proportional to its content in coal. The tendency of element leaching into diluted acid solutions was similar to that into chelating agent solutions.3. For the effective removal of Hg from coal, the heating of coal under nitrogen flow at 300-400℃ (mild pyrolysis) was proposed. Some coals showed high Hg removals of over 80%, whereas the Hg removal of some coals was very low. The difference in the Hg removal efficiency among coals was ascribed to that in the mode of occurrence of Hg in coal.4. It was proved that some hazardous elements, such as As, B, and Se, were easy to leach out from coal fly ash. When fly ash was subjected to a hydrothermal treatment, As was effectively removed from the ash. The wastewater obtained by the process was treated with an adsorbent, Al-loaded Shirasu zeolite, which had been newly prepared, resulting in the efficient removal of As from the wastewater.