Characteristics of PCDD/F Distributions in Vapor and Solid Phases and Emissions from the Waelz Process

Abstract

The Waelz process is a classic method used for recovering zinc from electric arc furnace (EAF) dusts containing relatively high concentrations of PCDD/Fs (polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans) as well as volatile metals, such as Zn, Pb, and Cu, and chlorine. As a result of the operating temperature in the cooling process and high carbon and chlorine contents, significant PCDD/Fs are formed in the typical Waelz process, causing public concerns regarding PCDD/F emissions. In this study, flue gas and ash samplings are simultaneously conducted at different sampling points to evaluate the removal efficiency and the partitioning of PCDD/Fs between the vapor and solid phases in the Waelz plant investigated. With the environment (temperature window, sufficient retention time, chlorine, and catalysts available) conducive to PCDD/F formation in the dust settling chamber (DSC), a significantly high PCDD/F concentration (1223 ng TEQ/Nm3) is measured in flue gas downstream from the DSC of the Waelz plant investigated. In addition, the cyclone and bag filter adopted in this facility can only remove 51.3% and 69.4%, respectively, of the PCDD/Fs in the flue gas, resulting in a high PCDD/F concentration (145 ng TEQ/Nm3) measured in the stack gas of the Waelz plant investigated. On the basis of treating 1 ton of EAF dust, the total PCDD/F discharge (stack gas emission + ash discharge) is 840 ng TEQ/kg EAF dust of the Waelz plant investigated. Because of the lack of effective air pollutant control devices for PCDD/Fs, about 560 ng TEQ/kg EAF dust are discharged via stack gas in this facility.

Fate of Polychlorinated Dibenzo-p-Dioxins and Dibenzofurans in a Fly Ash Treatment Plant

“…Thus, most of the PCDD/F mass was absorbed on the activated carbon combined with fly ash and was removed by the bag filters. The mean PCDD/F concentration of bag-filter ash reported by Chi et al 19 was 0.760 ng I-TEQ/g, which was 86% of magnitude lower than the results of this study (5.24 ng I-TEQ/g). This was because of the fact that, in the study of Chi et al, 19 there was no injection of activated carbon in front of bag filters.…”

“…The mean PCDD/F concentration of bag-filter ash reported by Chi et al 19 was 0.760 ng I-TEQ/g, which was 86% of magnitude lower than the results of this study (5.24 ng I-TEQ/g). This was because of the fact that, in the study of Chi et al, 19 there was no injection of activated carbon in front of bag filters. With respect to the congener profiles of bag-filter ash, which are shown in Figure Figure 3, b and c. For the stack flue gas and slag, the dominant congeners are OCDD, OCDF, and 1,2,3,4,6,7,8-HpCDF, respectively, as shown in Figure 3, d and e.…”

“…The total PCDD/F I-TEQ concentration in the stack flue gas was 7.73 ng I-TEQ/Nm 3 (RSD ϭ 2%). In this study, because of the injection of activated carbon powder at 40 kg/hr into flue gas, the stack flue gas concentration is much lower than 145 ng I-TEQ/Nm 3 reported by Chi et al 19 The mean total PCDD/F I-TEQ compositions in bag-filter and DSC ashes were 5.24 (RSD ϭ 17%) and 4.82 ng I-TEQ/g (RSD ϭ 45%), respectively, whereas that in the slag was 0.0025 ng I-TEQ/g (RSD ϭ 99%). The DSC operation temperature was approximately 550°C, close to the de novo synthesis temperature window (200 -400°C), 20 and resulted in high PCDD/F content of DSC ash.…”

The PCDD/F Removal Efficiency of a Medical Waste Incinerator Dual-Bag Filter System

“…Recent research (Gao et al, 2009) reported that the stack gas emissions of PCDD/Fs from fourteen MWIs in China, which had capacities ranging from 5 to 25 ton/d, exhibited a large variation in concentration (0.08-31.60 ng I-TEQ/Nm) (Chi et al, 2006). Evidently, adding activated carbon can significantly reduce PCDD/F emissions, although large variations in such emissions still remain a challenging problem.…”

Health Risk from Exposure to PCDD/Fs from a Waelz Plant in Central Taiwan

“…A Waelz plant was established in central Taiwan in 2000 to treat and decontaminate these hazardous wastes. The PCDD/Fs concentration measured in stack gas at the Waelz plant exceeded 190 ng-I-TEQ/N m 3 (Chi et al, 2006). To meet the stringent emission limit in Taiwan , installing additional air pollution control devices (APCDs) was necessary.…”