Fate of Polychlorinated Dibenzo-p-dioxins and Dibenzofurans during the Thermal Treatment of Electric Arc Furnace Fly Ash

Abstract

Thermal treatment is often employed to recover the metals contained in electric arc furnace (EAF) fly ash, which is considered a major source of PCDD/Fs. After the present treatment, the mass and volume of untreated material (EAF fly ash + cullet) were significantly reduced by 44.2 and 89.2%, respectively; meanwhile the density increased significantly by 476%. These results indicate that the mass and volume of EAF fly ash can be effectively reduced to benefit the further disposal in landfill. Additionally, this study also investigated the fate of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) during the thermal treatment. For the EAF fly ash with an original PCDD/F content of 1414 ng I-TEQ/kg, 99.58% solid destruction efficiency (SDE) was achieved with thermal treatment at 1450℃. The total PCDD/F I-TEQ contents in ingot and slag were thus reduced to 9.83 and 6.44 ng I-TEQ/kg, respectively. The residual PCDD/F I-TEQ content in slag was far below the soil disposal limit in Taiwan (1000 ng I-TEQ/kg). After PCDD/Fs were decomposed in a secondary combustion chamber at 1200℃, the thermal treatment without air pollution control devices (APCDs) displayed an overall 91.28% destruction removal efficiency (DREW/OAPCD). Furthermore, the total PCDD/F I-TEQ concentrations in the cooling unit, filter and PUF cartridge were reduced to 1340, 131 and 383 pg I-TEQ/Nm^3, respectively. Notably, their total amount, 1736 pg I-TEQ/Nm^3, exceeded the emission limit in Taiwan (400 or 1000 pg I-TEQ/Nm^3). The cooling unit and filter used as APCDs in this study could significantly lower the PCDD/F I-TEQ. The amount of residual PCDD/Fs captured in the PUF cartridge, was low enough (＜ 400 pg I-TEQ/Nm^3) to be directly emitted into the atmosphere. Consequently, the thermal treatment with APCDs in this study was able to effectively reduce the PCDD/F contents in EAF fly ash, while the metal contents were recovered as the ingot.