De novo Formation of PCDD/F during Sintering: Effect of Temperature, Granule Size and Oxygen Content

Abstract

Integrated iron and steel industry is the major industrial source of dioxins or, more precisely, of polychlorinated dibenzo-p-dioxins (PCDD) and dibenzofurans (PCDF). Their main source is the sintering plant (preparing iron ore fines as feed for the blast furnace) and EAF for steelmaking. The influence of temperature (300–600°C), feed granule size (4 fractions, from 0.5–1 to 4–8 mm) and oxygen content (5 to 15 vol.%) on PCDD/F-formation has been investigated during de novo tests, involving a feed composed from the various sintering raw materials in their typical proportions. These experiments were conducted using a lab-scale vertical tube reactor and PCDD/F in off-gas and residue were collected together for analysis. Some CuCl₂ catalyst was wetly added, to ensure that PCDD/F-formation activity was well measurable. The experimental results show that dioxins peak at 350°C, a granule size of 2 to 4 mm and the highest O₂ concentration tested (15 vol.%). Within each homologue group, the isomer signature has been further scrutinized, with special emphasis on the seventeen 2,3,7,8-substituted PCDD/F, as well as on the seven PCDD-congeners and two TCDF usually associated with chlorophenol precursor routes, with the purpose to throw more light on the mechanism of PCDD/F-formation. For the first time ever, a complete congener-specific analysis is presented for sintering effluent and discussed. From this study and a number of former and ongoing studies, it is clear that iron oxide is responsible for the high PCDF/PCDD-ratio and the relatively low level of chlorination of PCDF and PCDD.