Stabilization Of Heavy Metals In Municipal Solid Waste Incineration Fly Ash Using Thermal Treatment With Rice Husk Ash

In recent years,the amount of municipal solid waste incineration?MSWI?has increased rapidly in China,and MSWI fly ash from incineration progress was classified as hazardous waste due to contaminants enriched in it such as heavy metals and organic pollutants.Therefore,MSWI fly ash must be treated effectively before landfill.The medium-temperature thermal treatment based on silica-based materials has efficient stabilization for various metals other than Cr,low energy consumption and volatilization of metals.So this method has a broad prospect of application.In this study,this method was used to stabilize heavy metals in MSWI fly ash based on rice husk ash?RHA?.The effect of combustion conditions of rice husk,process parameters during medium-temperature thermal treatment and Si/Al mole ratio on heavy metals stabilization were investigated in this study.The changes of chemical speciation of heavy metals,crystal structure and microstructure during treatment were tested by BCR continuous extraction experiment,XRD and SEM to analyze the mechanism of the stabilization of heavy metals.The main conclusions of this paper are as follows:The combustion conditions of rice husk has an important effect on the reactivity of RHA and stabilization efficiency of heavy metals.When the rice husk combustion temperature was 500?,the leaching concentration of Pb and Zn were decreased gradually with the increasing of burning time.When temperature was 600⁸00?,the leaching concentration of Pb and Zn decreases first and then increases.When temperatures were600?,700?and 800?,burning time which has optimal efficiency of stabilizing metals was 2h,1h and 1h respectively.The leaching concentrations of Pb in fly ash treated by RHA-500-3 and RHA-700-1 were 5.293 mg/L and 5.142 mg/L,respectively,which were47.44%and 48.94%lower than that of raw ash,respectively.The leaching concentrations of Zn in these two samples were 2.678 mg/L,2.278 mg/L,respectively,which were which were3.04%and 17.52%lower than that of raw ash,respectively.The stabilization efficiency of heavy metals using RHA was mainly affected by the specific surface area of RHA and the crystalline distribution of SiO₂.The results of XRD and BET analysis showed that RHA prepared under 500?for 3h has ultra-high specific surface area and reactivity,in which SiO₂ existed mainly in amorphous state.That was favorable for the formation of silicates and stabilization of heavy metals.When temperature was above 600?,the stabilization efficiency of heavy metals decreased with burning time extension due to the increasing of crystal content of SiO₂ in RHA and the decreasing of specific surface area.Process parameters have an important impact on stabilization efficiency of heavy metals in MSWI fly ash using RHA.When thermal treatment temperature was 500?,the main products were silicates such as Ca₁₀?SiO₄?₃?SO₄?₃F₂),Ca₃SiO₅,which facilitated the conversion of acid-soluble and reducible states of Pb,Zn,Cu,and Cd to more stable states.Therefore,the leaching concentrations of Pb,Zn,Cu and Cd decreased to 0.086 mg/L,0.084 mg/L,0.737 mg/L and 0.084 mg/L,respectively.The crystallinity and type of silicates increased with thermal treatment time extension,which facilitated the conversion of acid-soluble,reducible and oxidizable states of Pb,Zn,Cu,Cr and Cd to more stable states.When the liquid-solid ratio increased from 0.5 to 0.9,the leaching concentrations of Pb,Zn,Cu and Cd in fly ash treated at 500?gradually decreased.The addition amount of RHA-500-3 in the range of 2%⁴%had ideal stabilization efficiency for Pb,Zn,Cd and Cu,and the leaching concentration was far below the national standard limit.When the addition amount of RHA-500-3 was above 6%,a large amount of SiO₂ existed as amorphous state,which was not conducive to the growth of silicate crystal structure and the stabilization of heavy metals.Medium-temperature thermal treatment for MSWI fly ash adding the mixture of RHA and secondary aluminum dross achieved efficient stabilization of heavy metals such as Cr,Pb,Zn,Cu and Cd.Under the conditions of adding 5%Si-Al materials?n?Si/Al?=2.5?and thermal treatment at 500?and 600?for 20 min,some aluminosilicate crystal phases were formed such as Gehlenite phase?Ca₂Al₂SiO₇?and Hibschite phase,Katoite phase.These aluminosilicate crystal phases that had three-dimensional grid structure facilitated the conversion of acid-soluble and oxidizable states of Pb,Zn,Cu,Cr and Cd to more stable states.