Study On The Migration And Transformation Of Heavy Metals In Sludge Incineration Process And The Leaching Characteristics Of Heavy Metals In Ash Residue

The economic,efficient,and safe disposal of municipal sludge is an important environmental issue facing China.Among many disposal technologies,sludge incineration technology,which has high weight and volume reduction efficiency and fast processing speed,has received increasing attention.However,the emission of toxic pollutants such as heavy metals generated during incineration has always been a concern,which is a bottleneck problem that restricts the promotion and application of sludge incineration technology.This article takes the municipal sludge of Guangzhou City as the research object and carries out sludge incineration experiments using a tube furnace.Equipment such as ICP-OES,SEM-EDS,XRD,and XPS are used to study the migration and transformation of heavy metals during sludge incineration and the leaching characteristics of heavy metals in ash.The experimental results show that the incineration conditions have an impact on the migration and transformation of heavy metals.As the incineration temperature increases（750-950℃）,the volatilization rate of Pb increases first and then decreases,reaching a maximum volatilization rate of 47%at 850℃,and the residual and organic-bound forms of Pb increase after incineration.The potential migration of Pb decreases.Low temperature is conducive to the volatilization of Pb-containing chlorides with lower boiling points,while high temperature easily leads to the reaction of Pb with silicoaluminate,forming Pb Al₂Si O₆,which has a fixing effect on Pb.For As,Cd,Zn,Cr,and Ni,the volatilization rates increase with temperature and the residual forms increase.The volatilization rate of Pb decreases with increasing moisture content,from 47%to 7%.The distribution of Pb forms changes slightly,while the volatilization rate of As decreases first and then increases,and those of Cr and Ni increase,and Zn increases slightly first and then decreases,and Cd decreases,Moisture reduces the content of free chlorine ions in the incineration system,inhibits the reaction of Pb,Cd,and Zn oxides with HCl,and reduces volatilization.For Zn and Cd,water and organic carbon generate reductive gases such as CO₂,reducing metal oxides to more volatile metals or suboxides.The volatilization rates of Pb,As,and Cd increase with increasing oxygen concentration,and the residual form of Pb increases,while those of Zn,Cr,and Ni increase first and then decrease.The increase in oxygen concentration promotes the oxidative decomposition of sludge and Pb,As,and Cd-containing organic matter.The lack of oxygen reduces the activity of chlorine ions,resulting in a decrease in the amount of chlorides produced and an increase in the volatilization rate.However,when the oxygen concentration is high,the reaction of Zn,Cr,and Ni to form their oxygen-containing compounds dominates.The oxygen-containing compounds are stable,causing the volatilization rate to decrease.The experimental results of the influence of adding calcium-based materials（CaO,Ca（OH）₂）on the migration and transformation of heavy metals show that the calcium-based materials promote the volatilization of Pb,and the volatilization rate increases with the amount added.The proportion of the residue state decreased with the increase of the amount of CaO and Ca（OH）₂added,respectively from 12-29%and17-32%.The proportion of organic binding state increased slightly,while the proportions of effective state and iron-manganese oxide state did not change much.The stability of Pb remained basically unchanged.Combined with SEM-EDS,XRD,and morphology analysis,it is concluded that Ca and Pb have a"competitive relationship"in the reaction system,and the addition of calcium reduces the retention effect of silicate on Pb,leading to a significant decrease in the residual form and an increase in the volatilization rate of Pb.After adding calcium-based materials,the volatilization rates of As and Zn first decrease and then increase,while those of Cd,Cr,and Ni decrease,and the residual forms of As and Zn increase first and then decrease,while the residual forms and water-soluble forms of Cr and Ni increase.Therefore,in the actual process of controlling heavy metal emissions,appropriate calcium-based additives should be added according to the characteristics of heavy metals.The experimental results of the leaching characteristics of heavy metals show that the leaching concentrations of As,Cd,Pb,Zn,Cr increase with increasing leaching time,while that of Ni reaches its maximum value at 16 h.The leaching concentrations of As,Cd,Pb,Zn,and Ni increase with increasing liquid-solid ratio,while that of Cr increases first,then decreases,and then increases.For the leaching solution p H,the leaching concentrations of the six heavy metals are relatively high under acidic conditions,and those of As,Cd,and Cr decrease with increasing p H value,while those of Pb,Zn,and Ni increase first and then decrease.The smaller the particle size of the ash,the higher the leaching concentrations of the six heavy metals.In addition,the higher the incineration temperature of the sludge,the lower the leaching concentrations of heavy metals in the ash.