Formation of Dioxin in Industrial Combustors and Pyrometallurgical Plants

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Abstract
The formation and destruction of PCDD/F in industrial and pyrometallurgical processes are investigated on the basis of a case study in a secondary copper smelting plant. PCDD/F data have been gathered at different positions of the smelting plant. Under the reducing conditions of the vertical shaft furnace mainly PCDD are formed and largely destroyed by thermal postcombustion. However, while the flue gases are cooled from 1100°C to 350°C the concentrations of PCDD and PCDF in flue gases are increased by a factor of 10 and 80, respectively. Laboratory testing of de novo synthesis on the fly ashes collected at several positions in the smelting plant shows that when the boiler ash is subjected to thermal treatment at 300°C for 2 h in air, the amount of PCDD and PCDF is increased by a factor of 1.85 and 2.5, respectively. Examination of the ash samples from the smelting plant using scanning electron microscopy (SEM) indicates complex mineralogies and crystal structures. Thermogravimetric analysis (TGA) of the ash samples reveals no significant catalytic carbon oxidation below 600°C. Comparison of PCDD/F homolog profile and congener pattern of the plant measurements and laboratory testing indicates that the PCDD/F homolog profile from de novo synthesis experiments resembles those of the stack gas emitted from the smelting plant. It is suggested from this preliminary investigation that PCDD/F formation in the low-temperature postfurnace region is a phenomenon occurring in a number of industrial and pyrometallurgical plants. Furthermore, the mechanisms of PCDD/F formation in these plants are basically similar, i.e., through the de novo synthesis from carbon structures present in fly ashes. Key words: Metallurgical processes; dioxin; furan; fly ash; de novo synthesis