Behavior of PCDD/Fs, PCBs, CBzs and PAHs during Thermal Treatment of Various Fly Ash from Steel Industry

The integrated iron and steel industry is the main source of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs), or, more briefly, of dioxins. These mainly arise from the sintering process but also from other operations, such as blast furnace, basic oxygen furnace and electric arc furnace. In this study, we investigated the contribution of fly ash from the above-mentioned operations to PCDD/Fs as well as other organic pollutants (polychlorinated biphenyls [PCBs], chlorobenzenes [CBzs] and polyaromatic hydrocarbons [PAHs]). The experiments were conducted using a lab-scale vertical tube reactor, and target pollutants in off-gas and residue were collected for analysis. The experimental results show that sintering fly ash generates far greater amounts of these pollutants than other fly ash, especially in terms of PCDD/Fs and dioxin-like PCBs, which are 2–3 orders of magnitude higher. The homologue group profiles of the target pollutants also show significant differences. Both the PCBs and CBzs are dominated by low-chlorinated compounds in all samples; however, the STA sample generated a much more high-chlorinated compounds. In addition, the correlations between various target pollutants based on all samples show that PCDDs are clearly related to the low-chlorinated PCBs and CBzs, and PCDFs are strongly related with high-chlorinated PCBs and CBzs. Within each homologue group of PCDD/Fs, the isomer signature was also further scrutinized, with special emphasis on the seventeen 2,3,7,8-substituted PCDD/Fs, as well as the seven PCDD-congeners and two TCDF usually associated with chlorophenol precursor routes, with the goal of shedding more light on the mechanism of PCDD/Fs-formation.