Identification the Content of the Windbox Dust Related to the Formation of Pcdd/fs during the Iron Ore Sintering Process

This study measured the contents of the ionic and elemental constituents, and the chemical compounds containing in windbox (WB) dusts collected from a commercial iron ore sintering plant, then the results were used to identify possible PCDD/Fs formation pathways in the WB. We found that the abundance in both KCl and NaCl may provide a suitable condition for de novo synthesis of PCDD/Fs in the WB. Al, Fe, K, Ca, and Pb were the top five contents in WB16 dusts, but the coexistence of the above five metal contents warrants the need for further investigation on their roles in PCDD/F formation processes in the future. Although a low concentration of Cu was detected, it might be of importance to PCDD/Fs formation inside the WB. A total of 29 chemical compounds were identified. Among them, several oxygenated organic compound might be associated with PCDD/F formations at the beginning stage, but the roles of aromatic oxygenates on the formation of PCDD/Fs required further investigation. Finally, possible de novo synthesis pathways of PCDD/Fs were proposed based on the above findings. However, the above pathways are required further laboratory studies for validation before possible formation suppression approaches can be determined.

Kasai and his colleagues (Kasai et al., 2001a) have investigated that the PCDD/Fs congener profiles of exhaust gases of windboxes.They found that the concentration of PCDFs were 10 times in magnitude higher than PCDDs.The above pattern is similar to that of PCDD/F emissions from municipal waste incinerators.The de novo synthesis has been considered as an important formation pathway of PCDD/Fs during the iron ore sintering process according to the results obtained from several field and laboratory investigations (Kasai et al., 2001a(Kasai et al., , 2001b;;Xhrouet et al., 2001;Cieplik et al., 2003;Kuzuhara et al., 2003;Suzuki et al., 2004;Kasama et al., 2006;Tsubouchi et al., 2006).For example, a series of laboratory sinter-pot studies have identified that the de novo synthesis could be occurred in the drying zone of the sinter bed (Cieplik et al., 2003).More recently, the concept of the two-stage release of PCDD/Fs from the rear half part of the sinter strand has been proposed by Kasama et al. (2003).Here, the formation of PCDD/Fs at the end of the wet zone and the beginning of the drying zone of the sintering bed was considered as the first stage.As the melting zone touching the bottom of the sintering bed (i.e., "the burn through point"), PCDD/F formation inside the corresponding windbox (WB) was considered as the second stage, and also the peak formation stage, of PCDD/Fs via the de novo synthesis process.In order to elucidate the de novo synthesis process occurred in WB, Tsubouchi et al. (2006) conducted a study focused on investigating functional groups containing in WB dust.Several functional groups, such as carboxyl, lactone/acid anhydride, and ketone/ether/phenolic groups, were identified by using temperature-programmed desorption (TPD) and temperature-programmed oxidation (TPO) techniques.Possible de novo pathways for synthesis of PCDD/Fs on the WB dust have been proposed based on their results.Briefly, the surface oxygen functional groups found in the WB dust may involve in the formation of PCDD/Fs by catalyzed carbon gasification under de novo condition in the WB towards the end of the sinter strand (Kasama et al., 2006).However, due to the limitation of both TPD and TPO, detailed information regarding the oxygenated organic compounds still remains unclear.In addition, beside the oxygenated organic compounds, the investigation of other contents containing in WB dusts might also be important for better understanding the de novo synthesis pathway of PCDD/Fs in the WB.
It is known that directly suppressing PCDD/F formations during the sintering process is considered as a better approach for reducing PCDD/F emissions than end-of-pipe treatment methods (Chen et al., 2008).Therefore, if the contents containing in WB dusts can be identified, then the information may help researchers to determine PCDD/Fs formation pathways and eventually to initiate potential suppression methods in the future.The aim of this research was set out to meet the above purpose.

Conducting WB Dust Sampling
The whole study was conducted in a commercial iron ore sintering plant located in southern Taiwan with a Dwight-Lloyd type sintering process.The sintering machine was installed with 18 WBs (inner diameter of 1,025 mm) with an effective sintering area of 315 m 2 and total air suction capacity of 11,000-12,000 Nm 3 /min.The raw mix materials used in this sintering process including blending ore, coke breeze, fluxes (marble limestone and serpentine), and return fines (minipellet, blast furnace returning fine, sintering plant returning fine, and slurry).
Dust samplings were carried out in the sixteenth WB (denote as WB16) of the sinter plant.The location of WB16 represents the phase of the "the burn through point" in the sintering process, and is the exact location for the occurrence of the second PCDD/F formation stage (i.e., the peak formation) in the de novo synthesis process (Kasama et al., 2006).
All WB dust samplings followed the modified USEPA method 5 (U.S. EPA, 2001).A silica glass microfiber thimble (25 mm O.D., 90 mm length, Whatman, Maidstone, UK) was used for collecting dust samples with a sampling velocity of 1.5 × 10 -2 Nm/dry min and sampling time of 1 hr.All samples were collected at the same sampling positions.Total ten dust samples were collected.Among them, four, three, and three samples were designated for the analyses of the ionic constituents, metal contents, and the types of functional forms, respectively.All silica glass microfiber thimbles were baked at 500°C for 24 hrs before sampling to eliminate the organic compounds originally containing in the filter.
For the determination of the concentrations of various elemental constituents, another three filter samples were used.Each sample was mixed with 10 mL acid mix (6 mL HCl, 2 mL HF, 2 mL HNO 3 ) and then placed for 30 min.The mixtures were subsequently pretreated by a microwave digestion system (MLS-1200 mega, Milestone, Italy) with programmed operation conditions set at 250 W for 6 min, 400 W for 6 min, 650 W for 6 min, and 250 W for 6 min.Sixteen elements (S, Cu, Ni, Cr, Mo, V, Ti, Co, Al, Pb, Zn, As, Fe, K, Mg, and Ca) were analyzed using inductively coupled plasma optical emission spectrometry (ICP-OES) (Optima 3300 DV, Perkin-Elmer, Norwalk, CT, USA).This method detection limits for sulfur and other analyzed elements were 5 ppm and 0.1 ppm, respectively.
To determine the chemical compounds containing in WB dust, three remained samples were pretreated by Soxhlet extraction process for continuously 24 hr using toluene as the solvent.The extracts were concentrated and qualitatively analyzed for chemical compounds using a gas chromatography-mass spectrometry (GC/MS) (HP 6890/5973, Hewlett Packard, Palo Alto, CA, USA) with a HP-5MS column (0.25 mm I.D., 60 m length, 0.25 μm film thickness, Hewlett Packard, Palo Alto, CA, USA).The splitless mode was used for sample injection and the oven temperatures were set at 100°C for 10 min, 10 °C/min to 280°C, and held for 20 min.The analyzer mode of the selected ion monitoring (SIM) was used with mass resolution of R = 1,000.The compound identification was done by spectrum matching and library (NIST05 MS) searching.All library-matched species exhibited a degree of match better than 90%.

Ionic Constituents Containing in WB Dusts
Table 1 shows the contents of ionic constituents containing in WB dusts.High Cl -and SO 4 2-contents were found among the analyzed anions in the present study.The above result was quite comparable to that conducted by Tsai et al. (2007), although their dust samples were collected from the stack rather than from the windbox.In principle, the high Cl -contents were quite expectable since the iron ore and coke used in this sintering process contains ~200 and 100 mg/kg of water-soluble chlorides (Nakano et al., 2005).But for SO 4 2-, its high content might be contributed mostly by the sulfur content containing in the fuels (such as coke oven gas (COG) and coke breeze for igniting the burner).The above inference could be supported by Hu et al. (2008).They estimated that ~90% of sulfur in the steel manufacturing process were derived from fuels.For a sintering process, sulfides could be decomposed and oxidized to form gaseous SO 2 or SO 3 .Subsequently, oxidation of SO 2 might occur at the particle surface to form SO 4 2- (Mamane and Pueschel, 1979).Therefore, it is no so surprising to see that abundant SO 4 2could be found in WB dusts.
For cation analysis, results show that the concentrations of both K + and Na + were higher than that of other analyzed cations.It might be because the involved raw materials for the iron ore sintering process, such as iron ore and return fines, were abundant in both K + , and Na + (Cieplik et al., 2003;Nakano et al., 2005).The above results were also comparable to that conducted by Tsai et al. (2007).Several laboratory studies have used alkali chlorides as raw material additives (act as catalysts and chlorine sources) to investigate the formation of PCDD/Fs during iron ore sintering process.They found that the alkali chlorides, such as NaCl and KCl, might catalyze the de novo synthesis (Kasai et al., 2001b;Cieplik et al., 2003;Kuzuhara et al., 2003;Nakano et al., 2005).Table 2 show the comparison between the amounts of alkali chlorides added in the above reference studies and the contents of K + , Na + and Cl -found in the present study.It can be seen that the concentrations of the contents of Na + , K + and Cl -found in this study were at least 88, 16 and 9 times in magnitude higher than that of previous studies, respectively.This result indicates that the abundance in both KCl and NaCl may provide a suitable condition for de novo synthesis of PCDD/Fs inside the WB16.

Elemental Constituents Containing in WB Dusts
Table 3 shows the content of elemental constituents containing in WB16 dusts.We found that Al, Fe, K, Ca, and Pb were the top five elements in the WB16 dust samples and they accounted for 98.6% of the total analyzed elemental constituents.The above result is quite comparable to the results conducted by Tsubouchi et al. (2006) and Tsai et al. (2007).In principle, high Al, Fe, K, Pb and Ca concentrations found in the WB dust samples might be because: 1. fluxes, iron ore fines and coke breeze used in the iron ore sintering process could contain a variety of impurities, including Al 2 O 3 , and K 2 O (Cieplik et al., 2003;Nakano et al., 2005;Sammut et al., 2010); 2. the use of limestone (CaCO 3 ) and dolomite (CaMg(CO 3 )) as fluxes in the sintering process (Cieplik et al., 2003); and 3. the return fine used in the sintering process usually contains Pb compounds (Sammut et al., 2010).Among these five metal contents, several studies have shown that the existence of Fe and K might lead to the increase in PCDD/Fs formation (Hinton and Lane, 1991;Nganai et al., 2009;Fujimori et al., 2009).To the contrary, the existence of Pb (chloride and oxide) and Ca (oxide) might inhibit the formation of PCDD/Fs (Qian et al., 2005;Fujimori et al., 2009).But for Al, conflict roles (i.e., either act as a catalyst or inhibitor) have been found in PCDD/F formation processes (Hinton and Lane, 1991;Gullett et al., 1992;Qian et al., 2005).In this study, the co-existence of the above five metal contents warrants the need for further investigation in their roles on PCDD/F formation processes.
In this study, the concentration of Cu found in WB16 dusts was 0.0308 mmol/g (Table 3).Although its concentration was much lower than that of Fe (6.69 mmol/g) and K (2.65 mmol/g), it was comparable to the result (0.0180 mmol/g) obtained from Tsai et al. (2005).The Cu presents in coal predominantly as chalcopyrite (CuFeS 2 ) which might decompose and react with sulfur Table 2. Amount of alkali chlorides (served as catalysts and chlorine sources) added in PCDD/Fs formation experimental studies and the ionic contents of K + , Na + and Cl -found in the present study.

Sources
Amount added Kasai et al. 2001 NaCl 0.08% (Na -0.317 mg/g, Cl -0.483 mg/g) NaCl 0.17% (Na -0.674 mg/g, Cl -1.03 mg/g) Cieplik et al., 2003 NaCl 0.0075 mg/g (Na -0.00297 mg/g, Cl -0.00453 mg/g) Kuzuhara et al., 2003 KCl (K -11.1 mg/g, Cl -10 mg/g) Nakano et al., 2005 NaCl 0.33 mg/g (Na -0.131 mg/g, Cl -0.199 mg/g) This study (ionic contents in the WB16 dust) Na + -59.6 mg/g K + -184 mg/g Cl --91.7 mg/g during the coke making process, then condense upon the surface of coke breeze (Coles et al., 1979;Khare et al., 2010).Therefore, Cu found in the present study might originate from the coke breeze which was used as a fuel in the sintering process.Table 4 compares Cu contents containing in the WB16 dust and that containing in the fly ash collected from different combustion processes.The Cu content in WB16 dust was higher than that of thermal power plant fly ash, but was close to the upper value of MSWI fly ash (Wiles, 1996;Aunela-Tapolaa et al., 1998;Ferreira et al., 2003;Nakano et al., 2005, Smolk-Danielowska, 2006;Sushil and Batra, 2006;Smichowski et al., 2008;Cobo et al., 2009).It is know that Cu plays a crucial role on the formation of PCDD/Fs in the fly ash for both the thermal power plant and MSWI.Moreover, laboratory study suggests that the small amount of Cu (0.0127 mg/g) could significant increase the formation of PCDD/Fs (Olie et al., 1998).Therefore, it could be expected that Cu might also play an important role on PCDD/Fs formation inside the WB.

Chemical Compounds Containing in WB Dusts
Table 5 shows the contents of chemical compounds containing in WB16 dusts.Eleven classes of organic compounds were detected, including alkane, alkene, aldehyde, alkyl benzene, alcohol, acid, ester, phthalate ester, phenols, phenoxy compound, and PAHs, above containing 3, 7, 1, 2, 2, 1, 4, 6, 1, 1, and 1 chemical compounds for each individual class, respectively.However, no chlorinated compound was detected might be because chlorinated compounds were presented in the gaseous form under either complete or incomplete combustion situations (Ismo et al., 1997).
Table 5 also compares the results obtained from the present study and that obtained from Tsubouchi et al. using TPD and TPO techniques (Tsubouchi et al., 2006).Noted that the study conducted by Tsubouchi et al. was not able to provide the detailed specific compound information due to the limitations of their adopted analytical methods.Therefore, only functional groups were provided in their Table 4. Comparisons of Cu contents containing in the investigated WB dusts of the selected iron ore sinter plant and those in the stack flue gas dust of an iron ore sinter plant, and in the fly ash of thermal power plants, MSWIs and hazard waste incinerator.

Sources Sampling location
As shown in Table 5, linear alkanes with high molecular weight (from C 15 to C 44 ) and a group of phthalate esters were found in WB dusts.These compounds might mainly come from lubricants (which were used in the sinter plant for lubricating the wheel bearing of the sintering strand) (Totten, 2006).The above inference can be confirmed by the detected compound of the 2,6-bis(1,1-dimethylethyl) -4-methylphenol (BHT) from WB16 dusts, since the BHT is widely-used anti oxidation additive in lubricants.
Several oxygenated organic compound were identified from WB16 dusts, including aldehyde, alcohol, organic acid, ester and phenols etc. (Table 5).According to mechanisms proposed by Tsubouchi et al. (2006), the O-containing species may play an important role on PCDD/Fs formation at the beginning stage.Moreover, several aromatic oxygenates, such as phenylmethanol, benzaldehyde, 2,6-bis(1,1-dimethylethyl)-4-methylphenol, and 1,3-bis[3-(phenoxy)phenoxy]benzene, were also observed in WB16 dusts (Table 5).These compounds were quite similar to those found in the MSWI fly (Jay and Stieglitz, 1995;Cains et al., 1997).The aromatic oxygenates could be degraded into phenols if the alkali and alkaline earth bases could also be found in the dust (Jay and Stieglitz, 1995).In the present study, although the co-existence of alkali metals and the aromatic oxygenates could be confirmed in WB16 dusts, no phenols were identified from the WB16 dust samples.It might be because phenols have a high reactivity on the carbon surface and hence they became undetectable in WB16 dusts (Jay and Stieglitz, 1995).Finally, it should be noted that the detected 1,3-bis[3-(phenoxy)phenoxy]benzene has a similar structure as that of diphenyl ether.Wiater and Louw investigated the reactions of diphenyl ether with Cl and Br atoms at 477°C (Wiater and Louw, 1999).Their results indicate that the phenoxy radicals can be formed via the reaction of diphenyl ether with chlorine atoms.Then, the generated phenoxy radical might react with another phenoxy radical to form dibenzofuran.Here, it should be noted that the temperature of WB16 is very close to 477°C.However, whether the aromatic oxygenates found in the present study will contribute to the formation of PCDD/Fs via the above pathway require further investigation in the future.

Possible de novo Synthesis Pathways of Organic Chlorine Compounds
According to theory proposed by Huang and Buekens, the gasification of carbon could be critical for the de novo synthesis in the low-temperature condition (Huang and Buekens, 1996).In principle, the gasification of carbon includes two steps: 1. the absorption of gases on carbon surface, and 2. the catalyzed carbon gasification.For the first step, the oxygen could dissociatively chemisorb on metallic sites and subsequently migrate to the carbon site to form complex at temperatures between 400500°C.Moreover, other researchers have shown that certain metals, such as Cu and Cr, can act as the dissociation center for the O 2 chemisorption (Mckee, 1970;Baker and Chludzinski, 1981).As described in Table 3, more abundant elemental metal, including Cu and Cr, can be found in the WB16 dust.In addition, the temperature of the flue gas in the WB at the latter phase of the sinter strand (i.e., WB16 in this study) usually falls to the range 400500°C.Particularly, abundant oxygen content could also be found in the flue gas due to the high suction air flow rate passing through the sintering bed.Therefore, the absorption of gases on carbon could be happened in WB16.This inference could be confirmed by the observation of several O-containing species, including carboxyl acids, alcohols, aldehydes, ketone, and esters, contained in the dust samples in the present study.For the second step, the metal mediated catalyzed carbon gasification takes place and the O-containing species are decomposed into carbon active sites and CO 2 /or CO.The activities of metallic oxides for carbon gasification, while presented in sequence, were: Pb > V > Mn > Co > Cr > Cu > MO > Ag > Cd > Fe > Pt > Ni > Zn (Mckee, 1970).According to the elemental analysis results of the present study, high concentrations of Pb, Cr and Cu were observed in the WB16 indicating that the occurrence of the gasification of O-containing species in the WB16 could be possible.In addition, the carbon active site could then react with HCl to form covalent C-Cl bonds (Takeda et al., 2006).Here, HCl could be formed during the combustion process since the Cl content was originally containing in the cokes (Kasai et al., 2001a).Yet, it is true that HCl might be converted to Cl 2 under the temperature range between 340 and 400°C via the Deacon reaction.Although Cl 2 measurements were not conducted in the present study, the result of high-level of Cl -content contained in the WB16 dusts (see Table 1) indicating that the Cl-containing intermediate could be formed in the WB16.Therefore, it infers that these Cl-containing intermediates could be further converted into organic chlorine compounds, including PCDD/Fs.Based on the above inferences, the proposed pathways for the de novo synthesis of organic chlorine compounds were summarized in Fig. 1.The further studies were needed to confirm the above inferences.

CONCLUSIONS
Based on ionic constituents found in WB dusts, our study suggests that the abundance in both KCl and NaCl may provide a suitable condition for de novo synthesis of PCDD/Fs in the WB16.After analyzing elemental constituents, we found that Al, Fe, K, Ca, and Pb were the top five contents in WB16 dusts.However, the co-existence of the above five metal contents warrants the need for further investigation on their roles in PCDD/F formation processes.Although the concentration of Cu was much lower than that of Fe, it might play an important role on PCDD/Fs formation inside the WB.In the present study, 11 classes of organic compounds were detected, and in total 29 chemical compounds were identified.Among them, several oxygenated organic compound may play an important role on PCDD/Fs formation at the beginning stage.On the other hand, whether the aromatic oxygenates found in the present study will contribute to the formation of PCDD/Fs required further investigation in the future.The results obtained from the present study can be used as a basis for laboratory studies to further elucidate possible formation pathways of PCDD/Fs in WBs in the future.

Fig. 1 .
Fig. 1.The possible de novo synthesis pathway of organic chlorine compound.

Table 1 .
Mean concentrations and their SDs (values in parenthesis) of ionic contents containing in the WB16 dust (n = 4).