Tracking of Chromium in Plasma co-Melting of Fly Ashes and Sludges

Leachable chromium in the incineration fly ash and wastewater sludge has been thermally stabilized by plasma melting at the temperature of 1,773 K. To better understand how chromium is stabilized with the high-temperature treatment, chemical structure of the slags sampled at temperature zones of 1,100–1,700 K has been studied by synchrotron X-ray absorption spectroscopy. The component-fitted X-ray absorption near edge structure spectra of chromium indicate that the main chromium compounds in the sludge and fly ash are Cr(OH)₃, Cr₂O₃, and CrCl₃. A small amount of toxic CrO₃ is also observed in the fly ash. In the plasma melting chamber under the reducing environment, the high-oxidation state chromium is not found. The slags in the plasma melting chamber have much less leachable chromium, which is due to chemical interactions between chromium and SiO₂ in the slags. The existence of the interconnected Cr-O-Si species is observed by refined extended X-ray absorption fine structure spectroscopy. In the Cr₂O₃ phase of the slags, their bond distances, and coordination numbers for the first (Cr-O) and second (Cr-(O)-Cr) shells have insignificant perturbation when experienced with different melting temperatures between 1,300 and 1,700 K. It seems that Cr₂O₃ and chromium encapsulated in the silicate matrix of the slags have relatively much lower leachability. With this concept, to obtain a low chromium leachability slag from the plasma melting process, the residence time of the melting chamber may be decreased, and the slag discharge temperatures may be increased to 1,300 K. This work also exemplifies utilization of molecule-scale data obtained from synchrotron X-ray absorption spectroscopy to reveal how chromium is thermally stabilized in a commercial scale plasma melting process.