Hot rolling sludge incineration: Suppression of PCDD/Fs by spent anion exchange resins

Steel industry is the main industrial source of 'dioxins', i.e. polychlorinated dibenzo-p-dioxins (PCDD) and dibenzofurans (PCDF). These arise mainly in the off-gas from sintering plant, yet also other operations, such as continuous casting, and hot and cold rolling could generate albeit minor amounts of dioxins. In this contribution it is verified in how far the incineration of hot rolling sludge (HRS) could contribute to such emissions and, if so, how these could be suppressed at low cost. Spent anion exchange resin (SAER) appeared as a dioxins suppressant during combustion tests of hot rolling sludge in a bench-scale tubular furnace. The reduction efficiency (RE) of PCDD/F and I-TEQ-generation attained 91.1% and 90.2%, respectively, with the addition of 2.5wt.% resin, and the highest dioxins RE reached 97.8% with 10wt.% resin addition. The PCDD/PCDF ratio raised markedly when adding resin, indicating stronger suppression for PCDF- than for PCDD-formation. However, further research is still needed to establish the effect of resin on PCDD/F formation and inhibition during full-scale experiments. The off-gas released by sludge and resin co-combustion was also monitored and the residue analysed by Energy Dispersive Spectrometer (EDS); the results revealed that the SO₂ concentration in the off-gas was higher during co-combustion than when burning HRS only. Likewise, the sulphur content in the combustion residue rises with resin addition. Still, the suggestion that suppression of PCDD/F formation should be ascribed to the poisoning of catalytic metal by S-containing compounds seems to be invalidated by the high initial S/Cl-ratio of HRS, even before SAER addition. These surprising findings were further analysed by a scrutiny of fingerprints obtained at the five distinct dosages tested. The relative amount of 2,3,7,8-PCDD/F was remarkably depressed by inhibitor addition and the relative importance of the chlorophenols route strongly expanded. Several reaction schemes, showing the results of further signature analysis are presented and are expected to throw more light on the mechanism of formation and its suppression.