Study On The Transformation Mechanisms Of Heavy Metals And The Cyclic Characteristics Of Molten Salts During The Molten Salt Thermal Treatment Of Municipal Solid Waste Incineration Fly Ash

Recently,the annual yield of municipal solid waste incineration（MSWI）fly ash in China has exceeded 6 million tons,and it increases rapidly with the promotion and application of waste incineration technology.Due to the high concentration of leachable heavy metals,dioxins,and of soluble salts,the treatment and disposal of fly ashes has been an important part of the waste incineration technology.Based on the leaching process in the wet-extraction method and the stabilization of heavy metals during the thermal treatment,a novel method,i.e.,molten salt thermal treatment is proposed to treat the three-high pollutants MSWI fly ash.Considering the high chlorides in most MSWI fly ash,a suitable molten chlorides system（Na Cl-Ca Cl₂）was established for the ash treatment.With the excellent heat transfer and reactivity of molten salts,simultaneous removal of some heavy metals and dissoved salts from fly ashes were achieved under the mild thermal treatment environment.The transformation mechanisms of typical heavy metals（such as Cu,Zn,Pb and Cd）and the cyclic characteristics of molten salts were also highlighted in the study.The main conclusions are showed below.Molten salt thermal treatment strengthened the chlorination and dissolution of heavy metals from fly ashes,and also improved the leaching stability of heavy metals in reacted slags.Compared with the traditional thermal treatment of fly ashes,molten salt thermal treatment promoted the chlorination of Cu,Zn,Pb and Cd at 600/800^oC,increasing the volatilization of most heavy metals more than 3 times.After the molten salt thermal treatment at 800^oC,more than 70%of total Cu,Zn,Pb and Cd were probably dissolved into the molten salts as chlorides,and the leachate concentration of heavy metals（especially Pb）in molten slags also reduced to below 1 mg/L.Oxide is one of the main chemical states of heavy metals in MSWI fly ash.Most Pb O/Cd O was directly chlorinated by the ionic Cl^-in molten salts at600^oC,while a small amount of Cu O/Zn O could be indirectly chlorinated by the volatilization and decomposition of molten chlorides at 800^oC.Moreover,Si/Al-compounds in fly ashes may enhance the release of chlorine-containing gases（such as HCl/Cl₂）from the molten salts,strengthening the indirect chlorination of Cu O/Pb O/Cd O.However,the morphological changes of Zn O were not obvious due to its poor reactivity of chlorination.SiO₂ and Al₂O₃ in fly ashes promoted not only the chlorination of heavy metals,but also the stabilization of Zn O.Under the single influence of SiO₂/Al₂O₃,molten chlorides were facilitated to release HCl/Cl₂,which promoted the indirect chlorination of Cu O/Pb O/Cd O.In contrast,the motivation of Al₂O₃ was weaker than that of SiO₂.Although SiO₂ promoted the indirect chlorination of Zn O in molten salts,it also reacted with some Zn O to generate Ca₂Zn Si₂O₇ at 600^oC.Meantime,Zn O could merely be converted into Zn₆Al₂O₉ by Al₂O₃.With the co-exist of SiO₂ and Al₂O₃,SiO₂ had a stronger effect on the morphological transformation of Zn O than Al₂O₃.The results of thermodynamic equilibrium calculations also supported the experimental reactions to some extent.Generally,there is a certain change in the content of SiO₂/Al₂O₃ in fly ashes,but the fluctuation had little effect on the chemical action of heavy metals in molten salts.However,the difference in the crystal form of the compounds may affect the reactions.In the recycling process of molten salts,the dissolution of some oxides and souble salts changed the physical compositions as well as some chemical parameters of the molten reagents.With the recycling of molten salts for the thermal treatment of MSWI fly ash for six times,Na Cl,KCl,Ca CO₃,Ca SO₄,SiO₂ and Ca（OH）₂ in fly ashes were gradually dissolved and enriched in the molten salts.The interactions between Na Cl/KCl/Ca CO₃/Ca SO₄/SiO₂ and molten salts resulted in a longer time for the melting of the reacted mixtures,with the increased melting enthalpy from 131.7 J/g to more than 180.8J/g.Meanwhile,the thermal stability of the molten salts was declined.Although molten salts could maintain the relatively low viscosity at high operating temperatures,the precipitation of Na Cl,KCl and Ca CO₃ in the cooling process might have a promotion effect on the viscosity value.Contrary to Ca SO₄,the dissolution of Na Cl,Ca CO₃,KCl and SiO₂ led to an increase in the specific heat capacity while a decrease in the thermal diffusion of molten salts.