Study On Migration And Transformation Of Heavy Metals During Co-sintering Of Fly Ash With Different Wastes

Incineration of waste generates a large amount of fly ash,which contains high levels of heavy metals and hazardous substances such as dioxins.The presence of sulfur/chlorine,minerals,and other components can affect the migration of heavy metals and gas emissions,with chlorine in particular having a corrosive effect that increases the difficulty of post-treatment of gases.Therefore,in order to achieve the goal of fly ash thermal treatment stabilization,resource utilization,and harmlessness,this paper investigates the influence of S/Cl and minerals on the mechanism of heavy metal migration.It uses methods such as washing to control the native Cl content of fly ash,soaking fly ash in sodium sulfide to add S,and explores the effects and mechanisms of S,Cl,and S/Cl on heavy metal migration.In addition,rice husk ash and secondary aluminum ash contain components such as Si and Al,rather than single compounds like Si O₂ and Al₂O₃.This paper also studies the effect of these wastes on heavy metal migration and gas emissions during fly ash sintering,and explores the possibility of co-treatment of fly ash using rice husk ash and secondary aluminum ash.Finally,thermodynamic simulation was performed using the FACTsage software to help understand the changes in heavy metal form and content during fly ash sintering.The main conclusions of this paper are as follows:（1）Temperature has a significant impact on the retention of heavy metals（Cu,Cd,Cr,As,Pb,Zn,Ni）and decreases their retention rate.Chlorides have a lower melting point,while sulfides have a higher melting point.Therefore,native chlorine promotes the volatilization of metal chlorides such as Pb,Cr,Cu,Zn,and Ni,while sulfur can significantly improve the retention of heavy metals.In addition,due to the influence of mineral reactions,the effects of sulfur and chlorine on Cd and Mn are relatively small.S/Cl interaction promotes the formation of Cr,Cu,Zn,Mn,and Ni oxides,increasing their retention rates.The individual effects of S and Cl on As are relatively small,but S/Cl interaction promotes the generation of As（g）,leading to a decrease in As retention.Individual S and Cl promote the formation of Pb Cl₂（g）and Pb S（g）,leading to a decrease in Pb retention.However,in promoting volatilization,Cl is more effective than S.Under the action of sulfur alone,most of the Pb exists in the form of residual Pb S（s）and Pb O（s）,so the addition of S during fly ash sintering actually improves Pb retention.However,excess S will reduce Pb retention.The thermodynamic simulation results agree well with the experimental results.The acid leaching degree of sintered residue of water-washed fly ash slightly increases,while the addition of S decreases the acid leaching of heavy metals after fly ash sintering.The synergistic effect of S/Cl significantly increases the acid leaching rate of As,Cd,Cu,Mn,Ni,Pb,and Zn.As sulfur decomposes Si-O-Si glass network and releases heavy metals,Cl accelerates the release of heavy metals,thereby increasing the acid leaching level.（2）Rice husk ash can promote the decomposition of Ca CO₃ and the emission of CO₂,S,and Cl during fly ash sintering.The Si O₂ in rice husk ash affects the mineral’s melting phase transition,promotes the production of Ca Si O₃,and redistributes the existence form of Cl.This leads to a reduction in Ca Cl₂,promotes the generation of other chloride forms（such as Na Cl and HCl）,and increases the content of low-boiling-point metal chlorides while decreasing the retention rates of Pb and Zn.The C in rice husk ash can reduce high-valence Cd to low-valence and easily volatile Cd,Cd O,and Cd OH,thereby reducing the retention rate of Cd.Additionally,rice husk ash can promote the generation of heavy metal oxides（such as Cu₂O（s）and Cr₂O₃（s））during fly ash sintering.However,if the content of rice husk ash is higher than 15%,it will increase the melting point of fly ash,which will negatively affect fly ash thermal treatment and flue gas emission treatment.（3）Secondary aluminum ash contains a large amount of Al₂O₃,which generates spinel-like materials such as Ca₁₂Al₁₄O₃₅,Ca₂Al₂Si O₇,etc.during high-temperature sintering of fly ash.Due to high-temperature ion effect,heavy metals such as Cu²⁺and Zn²⁺can replace Ca²⁺to form spinel-like materials such as Zn Al₂O₄,Al Ni₂O₄,Ni Al₂O₄,Cu Al₂O₄,Al Cr₂O₄（s）,thereby improving the retention rates of Zn,Ni,Cu,and Cr.Additionally,secondary aluminum ash also promotes the formation of heavy metal oxides（such as Cu O（s）,Ni O（s）,Pb O·Ca O（s））.However,as the secondary aluminum ash contains active metal Al,it affects the binding of heavy metal oxides with Al2O3,thereby promoting the production of heavy metal chlorides.Therefore,excessive secondary aluminum ash（50%）actually reduces the retention rates of Cr,Cu,Ni,Pb,Zn,and Cd.In addition,the addition of aluminum ash can reduce the volatilization of Na Cl（g）,thus suppressing the emission of Cl gas during fly ash sintering.Aluminum ash can also reduce the emission of sulfur in S gas because sulfur mainly exists in the form of Ca SO₄（s）in the residue.Therefore,adding 15%secondary aluminum ash during fly ash thermal treatment is helpful in reducing gas emissions while improving the solidification and stabilization of heavy metals.