Effect Of Mineral Additives On Heavy Metals Characteristics Of MSWI Fly Ash During Thermal Treatment

Municipal solid waste incineration (MSWI) fly ash, secondary pollutants duringincineration of MSW, contains high concentration of dioxins and leachable heavy metalssuch as Cd, Cr, Pb, Zn, which is regarded as potential threaten to human health andenvironment. Thermal treatment was considered as the best technique for disposal ofMSWI fly ash, but thermal treatment consumes a great deal of energy, and some heavymetals evaporate during the melting process. Additive plays a significant role in thermaltreatment to solve these disadvantages, which affects the melting behavior of fly ash andleaching behavior of heavy metals. Mineral additives, instead of pure chemistry reagent,were hardly studied to evaluate the effects. Besides, in glass production industry, mineraladditives were used as flux to reduce the melting temperature.In this paper, raw fly ash acquired from two of Wuhan incinerators, mixed withspodumene, fluorite, barite and lithium tetraborate, was studied in order to investigate themelting characteristics and heavy metals migration of municipal solid waste incineration(MSWI) fly ash during thermal treatment. Besides, thermodynamic calculation of heavymetals and melting kinetics was studied with the HSC software of TG-DSC experiment inthe interest of simulating the migration law of heavy metals and revealing the meltingmechanism.The experimental result on melting characteristics showed:1.Mineral additives helpsto reduce the flow temperature of fly ash, and therefore to decrease the energy consumptionduring thermal treatment;2.Mineral additives and treatment temperature are the maininfluence factors of crystalline structure and microcosmic morphology of slags, and mineraladditives can lead to formation of new crystal, which represents different crystalline in themicrocosmic morphology of slags, when the treatment temperature is lower than themelting temperature.The research on migration and leaching toxicity of heavy metals showed:1.MSWI flyash begins to melt in lower temperature with additon of mineral additives so that theevaporation of heavy metals decreases;2.Mineral additives contribute the slags to form inglass phase, which allows the heavy metals be fixed in the phase structure, resulting in redecing the leaching toxicity of heavy metals;3.The leaching toxicity of heavy metals inthe slags is much lower than the standard values, and therefore the slags can be disposeddirectly.HSC thermodynamic calculation results showed:1.the migration law of heavy metals,such as Cd、Co、Cr、Cu、Ni、Pb and Zn, is similar during thermal treatment. Heavy metalsexists in the form of sulfide or oxide below600-700℃, and starts to evaporate in the formof chlorides with different valence state when it comes to certain temperature;2.Someheavy metals reacts with other heavy metals resulting in new formation of complex.It is found in the TG-DSC experiment:1. weightless peak and endothermic peak of flyash moves forward in different degrees with mixture of different additives, and also meltingkinetics parameters changes along with the mix;2. Spodumene and lithium tetraborate canlower the kinetics parameters to reduce reaction resistance, while fluorite and bariteperform on the contrary.