Environ. Sci. Technol. 2004, 38, 4222-4226
Formation of PCDD/Fs in the Sintering Process: Influence of the Raw Materials C EÄ L I N E X H R O U E T * A N D EDWIN DE PAUW Mass Spectrometry Laboratory, University of Lie`ge, B6c Sart Tilman, B-4000 Lie`ge, Belgium
The sintering process is among the major sources of PCDD/Fs in the environment. This research studies the influence of the raw materials in this type of industrial plant on the amounts of PCDD/Fs generated. Particular interest is given to coke, which constitutes the principal source of carbon for the de novo synthesis of PCDD/Fs, and to the dust collected in the electrostatic precipitator (E.S.P. dust), usually recycled in the raw materials. The de novo synthesis of PCDD/Fs is simulated at the laboratory scale by thermal treatments of the samples. The use of a particular coke as a fuel does not drastically reduce the formation of PCDD/Fs. Actually, the global amounts of PCDD/Fs generated from the graphite and the two cokes tested are very similar. Only modifications in the fingerprint are observed. On the other hand, the addition of 10 wt % dust collected in the electrostatic precipitator leads to the formation of amounts of PCDD/Fs multiplied by a factor larger than 103. These results imply caution against the recycling of this E.S.P. dust in the raw materials.
Introduction Polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) are highly toxic compounds produced by some natural processes and different human activities, especially combustion processes. Reducing human risk toward PCDD/Fs implies a contamination monitoring of the food chain, but also the reduction of the emissions of these pollutants in the environment. Identification and control of the sources are thus essential. Anthropogenic sources of PCDD/Fs include the incineration of waste and most combustion processes. Since the discovery of PCDD/Fs in the flue gas and fly ash of municipal waste incinerators (1), strict emission limits have been set at the European level for these facilities, and improvements of the purification systems in these installations have strongly reduced PCDD/Fs emissions from incineration (2). Metallurgical processes, and in particular the sintering process, are still significant sources of these pollutants in the environment. In most European countries, the sintering process is now recognized as the major source of PCDD/Fs (3-5). The sintering process is an essential step in an integrated iron and steel plant. In this process, the iron ore is converted to larger lumps able to be charged into the blast furnaces. The sinter plant consists of a 50-100 m long, 3-5 m wide, horizontal strand, which supports the feed of hematite ores, * Corresponding author phone: +32-4-366-34-22; fax: +32-4366-43-87; e-mail:
[email protected]. 4222
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 15, 2004
coke breeze, and lime and moves slowly. Burners initiate the process by igniting the feed layer on top, and ambient air sucked through the layer moves the flame front downward. The sinter is then cooled, broken, and screened before being charged into the blast furnace. Separate chambers, called wind boxes and located below the strand, collect the off-gas prior to filtering in appropriate dust collectors (usually electrostatic precipitators). The detailed mechanism(s) as well as the spot(s) of PCDD/Fs formation in the sintering process remains unknown, although all of the necessary ingredients are present: carbon from the coke, oxygen in the air sucked through the cake, chlorine, and catalytic metals available in the ores. The European Commission points out this metallurgical process as the most worrying in terms of PCDD/Fs emissions and underlines the necessity of corrective actions (6). The huge amounts of fumes emitted (more than 106 Nm3/h) with high concentration of PCDD/Fs lead to a real need to reduce and control the discharge of these pollutants from this process. This requires the understanding of the formation spots and formation mechanisms of these pollutants in the process as well as the knowledge of the factors influencing this formation. The goal of this research is to point out critical elements that influence PCDD/Fs generation during the sintering process. The coke added as a fuel in the feed is the major source of carbon in the process for the de novo synthesis of PCDD/ Fs. Different authors have shown that the carbon morphology (amorphous-graphitic structure) is an important factor in the de novo synthesis of PCDD/Fs (7-13). The crystal lattice of graphite is obviously resistant to an attack of chlorine/ oxygen to a greater extent than other carbon structures, which consist in part of amorphous carbon and of microcrystalline carbon with a degenerated graphitic structure of disoriented layers. The way the coke is prepared (temperature and coking time) can certainly influence the structure of the coke, and thus perhaps its ability to produce PCDD/Fs by de novo synthesis. This study describes the thermal behavior of one graphite and two cokes differing by their coking time (9 or 15 h at 1325 °C) relating to the de novo synthesis of PCDD/Fs. The homologue and isomer distributions are also considered to attain a better understanding of the formation mechanism of these compounds. The sintering process also allows the recycling of various residues (iron containing dust and sludges). The dust collected in the electrostatic precipitator (E.S.P. dust) often constitutes a component of the raw materials. This E.S.P. dust presents a high capacity to form PCDD/Fs by de novo synthesis (14, 15, 17). This complex matrix contains carbon and chlorine as well as different metals supposed to catalyze the de novo synthesis of PCDD/Fs. The presence of recycled E.S.P. dust in the raw materials may increase the amounts of PCDD/Fs generated during the sintering process. In the present work, the recycling of E.S.P. dust is simulated by adding E.S.P. dust to the samples (cokes or graphite) before thermal treatment. Preliminary results of our investigation have been published previously (16, 17).
Experimental Section Materials. The following materials were used: a solution of 2,3,7,8-Cl-substituted 13C12-labeled PCDD/Fs (EPA 1613 LCS, Campro Scientific, Veenendaal, The Netherlands); toluene (p.a., Baker); hexane (p.a., Baker); dichloromethane (p.a., Vel); dodecane (Merck); sulfuric acid (95-97%, Baker); 10.1021/es034679t CCC: $27.50
2004 American Chemical Society Published on Web 06/29/2004
TABLE 1. Elemental Analysis of the Original E.S.P. Dust E.S.P. dust com position (in wt %) Mg Al Si P
S
Cl
K
Ca
1.04
2.17
3.62
0.24
4.07
9.55
9.07
7.83
Cr
Mn
Fe
Cu
Zn
Pb
C
0.04
0.31
49.90
0.17
0.34
5.98
3.34
performed. The native concentration was determined by isotopic dilution using the 2,3,7,8-Cl-substituted labeled PCDD/Fs to quantify all of the native isomers within homologues, assuming equal response for all isomers within an isomer group and no isomer-selective losses during the cleanup. Because 13C-OCDF was not present in the solution standard, this congener was not quantified. The different congeners were identified according to ref 18.
Results and Discussion sodium chloride (p.a., Merck); potassium hydroxide (p.a., Merck); sodium sulfate anhydrous (Baker); aluminum oxide (activated, neutral, type 507c, Aldrich); glass wool (DMCS treated, Alltech Europe); and technical dry air (Air Liquide, Belgium). Cokes. Two different cokes (Westfalen) and one graphite (Fluka, purum powder
