Study On A Low-Energy-Consumption Melting Solidification Method Of Municipal Solid Waste Incineration Fly Ash

Municipal solid waste incineration(MSWI)flying ash is a type of hazardous waste(HW18)because it contains harmful substances such as dioxins/furans and heavy metals.Among the treatment technologies of fly ash,the melting solidification method attracts aboard attention owing to its high volume reduction rate,stable slag properties,complete removal of dioxins and low dissolution rate of heavy metals.In order to solve the problems of high energy consumption and high equipment investment caused by high melting temperature of fly ash,the low-energy-consumption melting solidification method of MSWI fly ash was studied.First,the effects of different fluxes on the melting temperature and energy consumption of model ashes and the fluxing mechanism were studied.Then,the effects of additives on the distribution of heavy metals,the leaching of heavy metals from the slag and the mineral composition of fly ash during the melting process were studied using a real MSWI fly ash.Finally,the solidification mechanism of heavy metals at atom level based on density functional theory was disscused.The effects of different additives(SiO2,Al2O3,CaO and B2O3)on the melting temperature and energy consumption on fly ash were compared.The thermodynamic calculation results showed that compared with SiO2,Al2O3 and CaO,the addition of B2O3 can not only further reduce the melting temperature of fly ash,but also reduce the energy consumption.The optimum B2O3 addition amounts of different fly ash(B/A:1.22,2.61,4.48,6.43,6.90,8.32,8.82)are 142 g/kg,266 g/kg,272 g/kg,292 g/kg,285 g/kg,377 g/kg and 310 g/kg,respectively.The melting temperature of these ashes decreases by 904-1428? and the energy consumption is reduced by 19.5-75.4%.B2O3 and SiO2 can work together to reduce the melting temperature and energy consumption when B/A is high(2.61-8.82),and SiO2 and B2O3 can be introduced into the fly ash by adding waste glass and other boron containing waste to realize the coordinated disposal of waste.The melting characteristics of the multicomponent system of CaO-SiO2-Al2O3-Fe2O3-MgO under the influence of B2O3 were studied.The results showed that the flow temperature,the temperature corresponding to the-6%projection area reduction,and the minimum value decreased from 1450? to 900?,950? to 681?,and 1286? to 919?,respectively.The temperature at which the particles agglomerated decreased with an increase in the B2O3 content.Thermodynamic calculations combined with X-ray diffraction(XRD)were used to obtain the reaction mechanisms that contributed to the melting process.The results showed that B2O3 decreased the melting temperatures of the model ash samples,which could be related to the formation of the minerals Ca11B2Si4O22,Ca2B2O5,and CaB2O4 as well as the decrease of refractory minerals such as melilite.In additon,the feasibility of adding B2O3 into MSWI fly ash during melting treatment process was discussed.Ash fusion temperature(AFT)test,atomic absorption spectroscopy(AAS),X-ray diffraction(XRD),scanning electron microscope(SEM)and thermodynamic calculation were carried out.The results showed that the flow temperature decreased from 1211? to 986? with an increase in the B2O3 content from 0 to 15 wt%.The melting slag led to a decrease in the surface area and an increase in the mass transfer resistance of heavy metals.Thus,the agglomeration of the fly ash particles became easier with the reduced viscosity of the liquid slag.The volatilization of Pb,Cd and Zn in fly ash was inhibited.The leaching behavior of heavy metals was evaluated,the results indicated that Zn and Cu were stable whereas the Pb and Cd were soluble in glassy slag.Therefore,a further study on adding a moderate amount of SiO2 to adjust the ratio of Si,B,Ca and Al and improve the chemical stability of heavy metals should be considered.Based on density functional theory,the immobilization of Cd,Pb,Zn and Cu doping in Ca3(BO3)2,Ca2B2O5 and CaB2O4 were comparatively studied.The defect formation energies for substitutional doping model are higher than interstitial model which is the more favorable immobilization model for Cd,Pb,Zn and Cu,and the Ca2B2O5(13)doping model is the easiest to be formed.In the substitution doping models,the O atoms around the heavy metals are closer to them due to the effective ion radii of Cd,Zn and Cu are less than that of Ca.On the contrary,the effective ion radius of Pb is larger than that of Ca,and the atoms around it are crowded out.Cd,Pb,Zn and Cu can replace Ca to balance the electronegativity of O atom and form chemical bonds with the surrounding O atoms.Except for Pb-O,and the covalency of Cd-O,Zn-O and Cu-O is stronger than that of Ca-O.In addition,the order of covalency between Pb-O?Cd-O and Zn-O is as follows:Ca3(BO3)>Ca2B2O5>CaB2O4.In the interstitial doping model,the atoms around the heavy metals are crowded out,lattice constants and volumes increase.Bonds of O-Cd/Pb/Zn exhibited negative population values,revealing that filling of electrons into the antibonding states was the major orbital contribution.Cu-O exhibited negative population values mean that chemical bonds are formed.In the CaB2O4(?)doping model,B 2s and 2p orbital and Pb 6p,Zn 4s and Cu 4s orbital form hybridization near the Fermi level.