Dioxins from thermal and metallurgical processes: recent studies for the iron and steel industry

In thermal metallurgical processes such as iron ore sintering and metal sme lting operations, large flows of off-gases are generated. Mainly due to res idue recycling in such processes, chlorine and volatile organics are always present in the: feed. As a consequence of "de novo" formation, the off-gas es from such processes typically contain dioxins in the range 0.3-30 ng I-T EQ/Nm(3). So far there are only very few studies about the mechanisms of di oxin formation and destruction in these metallurgical processes. In an Euro pean Union (EU) research project "Minimization of dioxins in thermal indust rial processes: mechanisms, monitoring and abatement (MINIDIP)", integrated iron and steel plant has been selected as one of the industrial sectors fo r further investigation. A large number of particulate samples (feed, belt siftings, electrofilter) were collected from the iron ore sintering install ations from various steel plants and analyzed fur their organochlorocompoun d contents. Measurable amounts of PCDD/F, PCBz, PCB were found for all samp les. The various parameters influencing their de novo synthesis activity we re also evaluated in laboratory experiments, and such activity was found to be moderate for samples from the ore sinter belt, but extremely high fur s ome ESP dusts. Fine dust is active in a wide range of temperatures starting at 200 degreesC and declining above 450 degreesC; the optimal temperature for de novo synthesis was found to be around 350 degreesC; some inhibitors, such as triethanolamine, may reduce de novo activity by 50%, and lowering the Oz concentration in the gas stream leads to a much lower amount of PCDD /F formation. On the basis of their relative mass, typical operating condit ions and specific activity of the different samples, the regions in the sin tering plant where de novo synthesis may take place were tentatively establ ished. (C) 2001 Elsevier Science Ltd. All rights reserved.