Transformation and stabilization of heavy metals in hazardous waste incineration fly ash treated by NaCl-CaCl₂molten salt thermal process
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Graphical Abstract
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Abstract
Hazardous waste incineration fly ash was thermally treated using NaCl-CaCl₂ molten salt at relatively low temperatures (600–700°C). The effects of treatment temperature, duration, and molten salt/fly ash ratio on the distribution and leaching of heavy metals in the treated products were investigated to explore the transformation/stabilization mechanisms of heavy metals. The results indicate that heavy metals in the fly ash were directly or indirectly chlorinated via the molten salt or chlorides (e.g., NaCl, KCl) present in the fly ash, subsequently enriching in the molten salt phase, while the remaining metals were retained in the slag. The proportion of heavy metals in the molten salt was significantly correlated with their inherent volatility and the treatment temperature. After treatment at 600°C for 1 h with a molten salt/fly ash ratio of 1:5, the proportions of Cu, Zn, Cr, Cd, and Pb in the molten salt reached 84.45%, 82.28%, 75.35%, 93.08%, and 96.39%, respectively, accompanied by a 63% weight reduction. Using the sulfuric acid-nitric acid leaching method, the leaching rates of Cu and Pb in the slag decreased by up to 88.4% and 97.3%, respectively, while Zn, Cd, and Cr were undetected in the leachate, meeting the entry standards for hazardous waste landfills. With the acetic acid buffer solution method, the leaching rates of Cd, Cr, Cu, Pb, and Zn in the slag decreased by 99.8%, 90.5%, 98.4%, 79.6%, and 99.9%, respectively. Furthermore, adding CaO (4%) to adjust the NaCl-CaCl₂ molten salt system enhanced the stability of Pb in the slag.
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