Theoretical And Experimental Study On Solidification Of Heavy Metals By Plasma Melting MSWI Fly Ash/Red Mud

In recent years,municipal solid waste generation in China has increased rapidly.Incineration has become the main disposal method of MSW because of its huge mass and volume reduction.Municipal solid waste incineration（MSWI）fly ash is usually regarded as hazardous waste because of the high amount of heavy metals and organic pollution,which has become an urgent problem to be solved.Plasma vitrification is a highly efficient method for the disposal of MSWI fly ash.However,the high energy consumption,high cost of flux addition,and the enrichment of heavy metals in secondary fly ash prevent the application of plasma vitrification in MSWI fly ash treatment.In this paper,red mud is used as an additive to mix fly ash with red mud to reduce the energy consumption of plasma vitrification.Besides,the partition characteristics of heavy metals during the melting process of MSWI fly ash with red mud were carried out using a lab-scale plasma furnace between 1350℃to 1600℃under various Fe₂O₃/（Ca O+Si O₂+Al₂O₃,CAS）ratios.（1）The melting temperature of fly ash is related to the liquid content in the system.With the increase of the amount of red mud,the melting temperature of fly ash gradually decreases.When the ratio of raw material is Fe₂O₃/（Ca O+Al₂O₃+Si O₂）=0.1,0.2,0.3 and 0.4,the system melting temperature can reach 1600℃,1600℃,1500℃and1350℃respectively.（2）Based on thermodynamic equilibrium calculation,Factsage is used to simulate the migration process of heavy metals in the melting process.The results show that Pb,Zn,Cd and Cu tend to transfer to volatile gases with the increase of temperature,Zn,Cu,Cr and Ni are tend to react with Fe₂O₃ and Al₂O₃ to form solid phase with a relatively high melting point,and Pb is tend to react with Si O₂ to form Pb₂Si O₄ with low melting point.（3）The addition of red mud can improve the solidification rate of Cr,Cu,Ni and Zn,Fe₂O₃/CAS=0.1,and the solidification rate of Cr reaches the maximum value of92.43%at 1200℃;When Fe₂O₃/CAS=0.3 and the temperature is 1500℃,Cd and Pb reach the maximum of 7.91%and 13.4%respectively.When Fe₂O₃/CAS=0.4,the curing rate of Ni reaches the maximum of 97.12%at 1500℃.The comprehensive evaluation shows that when Fe₂O₃/CAS=0.3,the melting temperature is low,and the solidification effect of heavy metals is good.The solidification rates of heavy metals Ni,Cu and Zn increased by 33.0%,44.0%and 25.3%respectively..（4）When Fe2O3/CAS=0.3 and 0.4,the mass ratio of fly ash to red mud is 6.36:3.64 and 5.19:4.81 respectively,it can effectively inhibit the volatilization and enrichment of Ni,Cr and Cu in secondary fly ash.At the same time,the average enrichment indexes of Ni,Cr and Cu secondary fly ash are lower than 0.68,0.18 and2.4,far less than 1.1,0.8 and 9.7 when the waste incineration fly ash is completely melted.（5）The leaching toxicity results of molten slag show that when red mud and fly ash are treated by plasma co melting,the leaching concentrations of Cd,Cr,Cu,Ni,Pb and Zn meet the standard.The leaching rate of Cd is reduced by 85.0%-99.7%,the leaching rate of Cr is reduced by 98.1%-99.6%,the leaching rate of Pb is reduced by65.2%-100%,and the leaching rate of Ni increases slightly with the addition of red mud;The acid loss rate of molten slag is less than 2%,and the water absorption is less than 1.5%,which meets the technical requirements of solid waste vitrification products.