Migration Characteristics Of Heavy Metals During Mswi Fly Ash Thermal Treatment With The Hot Blast Furnace Slag

Due to the enrichment of heavy metals,dioxins and other hazardous substances,municipal solid waste incineration（MSWI）fly ash will seriously threaten environmental safety and human health,so it must be safely and harmlessly treated.Among the existing harmless technologies,heat treatment technology can degrade persistent organic pollutants and solidify heavy metals.Aiming at the problem of high energy consumption in heat treatment of MSWI fly ash,this paper proposes to use hot blast furnace slag to dispose MSWI fly ash,which realizes the utilization of sensible heat resources of blast furnace slag and the safe and harmless disposal of MSWI fly ash.The main contents of this paper are as follows:Firstly,the effect of temperature on the migration and distribution characteristics of heavy metals during MSWI fly ash heat treatment with the blast furnace slag was studied.The results showed that with the increase of temperature,the distribution of Cd,Cu,Pb,and Zn in MSWI fly ash was dominant in the secondary fly ash,and most of Cr was distributed in the residue.The proportion of heavy metals in the gas phase was less than 1%.XRD analysis showed that the secondary fly ash was mainly composed of Na Cl and KCl,with a small amount of Pb Cl₂,Zn Cl₂,Zn S and K₂Zn Cl₄,indicating that heavy metals mainly migrated in the form of chloride.Thermodynamic equilibrium calculations showed that more than 99%of Cr existed in the residue in the form of Mg Cr₂O₄ and Ca Cr₂O₄.Cd,Cu,Pb and Zn mainly migrated to the secondary fly ash in the form of Cd Cl₂（g）,Pb Cl₂（g）,（Cu Cl）₃（g）,Cu Cl（g）and Zn Cl₂（g）.The ability of hot blast furnace slag to dispose of different MSWI fly ash was compared and analyzed by thermodynamic equilibrium calculations.The results showed that under the condition of using only the sensible heat of hot blast furnace slag,the maximum MSWI fly ash disposal capacity of hot blast furnace slag was4%～8%,and the highest capacity is achieved when the alkalinity of disposal fly ash is close to 1.The maximum amount of MSWI fly ash can be up to 49%after preheating treatment.The preheating treatment and adjustment of MSWI fly ash alkalinity can significantly increase the MSWI fly ash consumption.The effects of MSWI fly ash doping on the leaching toxicity and migration characteristics of heavy metals in slag were investigated by experiments and thermodynamic equilibrium calculations.The results showed that the leaching toxicity of Cd,Cr,Cu,Pb,and Zn from the slag was lower than the national standard when the fly ash dosing did not exceed 40%.The effect of MSWI fly ash doping on the migration of Cd and Cr was not significant,and the volatilization rate of Cd was greater than 90%and the solidification rate of Cr was more than 95%.Thermodynamic equilibrium calculations showed that Cr existed mainly in the form of Cr₂O₃（slag）and Cd in the form of Cd（g）and Cd O（g）.The increase of fly ash doping promotes the migration and volatilization of Cu,Pb and Zn into gaseous chloride form,and the chlorine content is the main reason to promote the migration and volatilization.Finally,a research on the formulation of additive package to reduce the melting temperature of MSWI fly ash was conducted.The results showed that reducing the alkalinity could reduce the MSWI fly ash melting temperature,but the temperature reduction ranged from 50 to 100℃.The addition of boron-containing compounds significantly reduced the MSWI fly ash melting temperature.Under the suitable ratio,MSWI fly ash melting can be achieved at 850℃,and the heavy metal content of leachate of MSWI fly ash melt does not exceed the standard,which is lower than the limit value of national standard.