CHARACTERIZATION OF HEAVY METALS IN FLY ASH AND BOTTOM ASH UNDER CONTROLLED INCINERATION OF PLASTIC WASTES

摘要

A batch-type incinerator was used for the investigation of heavy metals emission from four plastic wastes incineration under controlled conditions including temperatures, air-supply flow rates and the concentration of oxygen. Samples were collected from the stack flue gas and bottom ash, then both were digested and analyzed by an inductively coupled plasma atomic e-mission spectrometry (ICP-AES). The results show that mean composition of Cd, Zn and Pb were 14.0, 19.5, 10.7 mg/g-particulate, respectively. The total contents of metal was 55.0 mg/g-particulate (excluding Al, Fe and Mn) in the stack flue gas for PVC, approximating an order of magnitude higher than those of LDPE, HDPE and PP wastes. In bottom ash, Cd and Zn are only 0.29 and 0.043 mg/g-ash for PVC, respectively. This is associated with the fact that the presence of high concentration of chlorine will produce high volatile chloride in the flue gas. Due to the influence of stabilizers and/or pigments, the summation of the total emission factor (both the flue gas and the bottom ash) is 887.0 mg/kg-feedstock for PVC wastes, approximately 2～10 times level of those for LDPE, HDPE and PP. On the basis of the level of vaporization, the elements are categorized as (ⅰ) high volatilization metals, including Pb, As and Cd; (ⅱ) medium volatilization metals, including Cu, Zn, Ni and Al; and (ⅲ) low volatilization metals, including Cr, Fe and Mn. In this study, indicatory metals were used to differentiate the compositions and to compare with the major components-emission factor higher than 10 mg/kg-(waste or fuel) for pollution source identification. Pb and Zn were the same elements to be indicatory metals for LDPE, HDPE and PP wastes, while Cd and Pb are the major elements and indicatory metals simultaneous for PVC. This study reveals that, in Taiwan, V and Ni are the same indicatory metals for diesel blank, diesel boiler and heavy boiler, same as U.S. EPA has suggested.