Reserach Express@NCKU - Articles Digest

The iron and steel industry produces important materials for automotive, construction and consumer product applications. But it is also one of the most energy-intensive industries and produces significant pollution emissions. The major operations of an integrated iron and steel industry include coke making, sintering, iron making, steel making, and rolling. Figure 1 presents the main processes of the integrated iron and steel plant in Taiwan.

Iron ore is the raw material, which consists mainly of hematite (Fe2O3) and magnetite (Fe3O4) or goethite (FeOOH). The sinter process, coke making, the heating furnace and the blast furnace are the major air pollution sources which contribute over 90% of the emissions from the integrated iron and steel plant. Coke ovens also emit a lot of PAH.

This study sampled the particles emitted from these processes and idenyified the constituents as the fingerprints of various emission sources. There were 21 element species, 11 ionic species, elemental carbon, organic carbon and 16 polyaromatic hydrocarbons (PAHs), measured to create “fingerprints” in these processes.

The elemental compositions of particles for the four processes are shown in Table 1. Sulfur, iron, and sodium are the major elements in the coke making process. The contribution of K and Pb is higher in the sintering process than in the other processes, especially the K contribution (157 mg/g), which is about 15% of the particle mass in the sintering process. Furthermore, S, Fe, Na, K and Ni contribute 147 -21 mg/g, to the particles in the cold forming process. In hot forming, S, Fe, Na and Ca are the major particle elements (60-14 mg/g). In general, sulfur had a higher mass contribution than the other elements, which resulted from the use of coal, flux, heavy oil, and many recycled materials in the iron and steel plant. The particle mass contribution of potassium in the sinter plant was higher than in other processes; this may be attributed to the lower boiling point and volatility of potassium. In addition, many recycled materials were fed into the sinter plant, causing a high concentration of potassium in the particle phase.

The sulfate concentration is higher than in other ionic compounds, with concentrations ranging from 122 to 349 mg/g in the four processes. This may be due to the sulfur content in the raw materials, which oxidizes to form SO2 and adsorbs or deposits on particles and then forms sulfate. In another pathway, SO2 may be catalyzed (e.g., Fe, Ni, etc.) to form sulfate in the particle phase. There is a substantial sulfate contribution to the particle mass. In addition, the concentrations of K+ and Cl- were higher than that of S (199 and 173 mg/g, respectively), in the sintering process. The return fines from the sintering plant, blended ore, serpentine, limestone and coke breeze were also recycled and mixed to make sinter. A large amount of recycled material is used in sinter plants, which may contain and potassium chloride compounds.

The elemental carbon ranged from 9 to 137 mg/g and organic carbon from 16 to 64 mg/g; the total carbon content ranged from 40 to 170 mg/g. The low carbon content was due to the high temperature (> 850 oC) manufacturing processes and the carbon burnoff. The coke making and cold forming processes revealed high carbon content, which was caused by the reduction reaction of coke formation and low thermal temperature treatment for the cold forming process.

Sixteen PAHs were analyzed in this study, and eight PAH compounds were detected in the four processes (Table 2). The sequence of PAH content in the particle phase was as follows for the four processes: cold forming (6.2 mg/g) > coke making (1.8 mg/g) > hot forming (1.6 mg/g) > sintering (0.5mg/g). This may be due to the oil added into the cold-rolled process and the lower operating temperature, which enhances PAH formation. Figure 2 shows the ring distribution of the PAHs. The percentage of the mass contribution of the 16 PAHs was 3-ring (88-69%)> 5-ring (3-13%)> 4- ring (8-24%) in this study. AcPy (3 ring) was the predominant PAH species. In addition, the carcinogenic compound Benzo(a)pyrene (BaP) was detectable only in the sintering process (0.041 mg/g). High carbon content was found in the coke making and cold forming processes as a result of the high PAH content of the two processes.

This study provides the detailed particle compositions and the baseline information on emissions from integrated iron and steel facilities. Detailed particle composition can be used as a “fingerprint” to identify the contribution from various pollution sources to the airborne particle in the vicinity of an integrated iron and steel plant.

Table 1 Elemental concentration in particulate emissions from various processes in steel plant

a mean value ± standard deviation
bND: not detectable
cEach processor took 3 samples and each sample took 3 duplicate analyses.
dMDL: Method detection limit; Al, Ca, Fe, K, Mg, Na, and S were measured by ICP-AES and Co, Zn, Pb, Cu, Sr, As, Cr, Ba, Mn, Ni, Se, Cd, Sb, and V were measured by ICP-MS.

Table 2 PAHs concentrations in the four processes

*ND: not detectable
** Naphthalene (NaP), Acenaphthene (Acp), Anthracene (Ant), Fluoranthene (FL), Benzo(a)anthracene (BaA), Indeno(1,2,3-cd)pyrene (IND), Dibenzo(a,h)anthracene (DBA), and Benzo(g,h,i)pyrene (BghiP) were not detected in these samples.
*** Each processor took 3 samples and each sample took 3 duplicate analyses

Figure 1 The main processes of the integrated iron and steel plant

Figure 2 Distribution of various classes of PAHs