Volatilization Characteristics Of Typical Heavy Metals During Ceramsites Sintering From Electroplating Sludge

Electroplating sludge usually contains high concentration of heavy metals,safe treatment and disposal of it has become an urgent need of electroplating industry.Due to the restriction of the number and space of hazardous waste landfill site,and the risk of metal release exists in the later stage of landfill,the resource utilization of electroplating sludge is the main way of treatment and disposal in the future.Electroplating sludge is mainly composed of silicate,and the utilization of building materials is an important potential way.Previous studies have confirmed that ceramsite firing may be an important utilization.However,the volatilization behavior and release mechanism of heavy metals during ceramsite firing from electroplating sludge are less studied.In order to understand the release behavior of heavy metals in firing process,the volatilization characteristics and kinetics of heavy metals in sludge with different Si/Al ratios and the presence of sulfide,chloride and oxide were studied in this paper,In order to provide scientific basis for the prediction of heavy metal pollution emission in firing process.In this study,the volatilization characteristics of Zn,Pb,As and Sb were explored by firing sludge with different Si/Al ratios.In order to facilitate expression,this paper described and discussed As by heavy metal.The volatilization rate of heavy metals increases gradually with the increase of temperature and reaction time in the process of low Si/Al ratio sludge firing.When the sintering temperature was 1150℃,the volatilization of heavy metals was the highest,the order of volatility was Pb>As>Sb>Zn.Compared with 700℃,the volatilization rates of Pb and As increased gently with time at 1150℃.XRD analysis showed that at 700℃and 800℃,Zn₂（Se O₃）Cl₂,Zn₂Co₃（OH）₁₀·2H₂O and Pb₄（NO₃）₂O₃decompose into Pb O and Zn_2.5VMo₈,which existed in ceramsite.When the temperature increased to 1150℃,most of Pb was transferred to fly ash and flue gas.Zn mainly existed in ceramsite as refractory oxides.SEM analysis results showed that ceramsite melts at1150℃,which encapsulated heavy metals and inhibited the volatilization of heavy metals.The volatilization of Zn,Pb,As and Sb at sintering temperature of 1150℃in ceramsite firing with high Si/Al ratio was obviously lower than that at low Si/Al ratio.SEM analysis showed that there are more molten glass phase substances in the ceramsite with high Si/Al content at 1150℃,which enhanced the wrapping effect of various heavy metals and inhibited the movement of heavy metals.This study also based on the analysis of the volatilization of heavy metals in the sintering process of sludge with different Si/Al ratios for kinetic analysis,obtained a comparison of the influence of Si/Al ratios on the volatilization kinetics of heavy metals,and determined the volatilization kinetics equation of each heavy metal.In order to understand the influence of different substances on the volatilization of heavy metals in ceramsite produced by electroplating sludge,Na Cl,Na₂S and Ca O were added to ceramsite raw materials for sintering experiments in this study.When Na Cl was added to low Si/Al sludge,Pb volatilization decreased significantly at 700℃,but the effect of Na Cl on Pb volatilization decreased with the increase of temperature.At 1150℃,the volatilization rates of Zn and Sb decreased by 4.42%and23.2%,and the volatilization rates of As increased by 5.02%.After Na Cl was added to high Si/Al sludge,the change trend of Pb was the same of low Si/Al ceramics.The volatilization rate of Zn increased by 0.8%,and that of As and Sb decreased by 18.5%and 30.2%.XRD analysis showed that Ca Pb₃Al₂Si₁₀O₂₄（OH）₆was formed at 700℃and 800℃,while Ca Pb₃Al₂Si₁₀O₂₄（OH）₆was decomposed at 1150℃.Therefore,the effect of Na Cl on Pb decreased with the increased of temperature.At 700℃,Zn was mainly composed of Zn₅（NO₃）₂（OH）₈·2H₂O existed in the ceramsite,and decomposition reaction and oxidation reaction occured with the increase of temperature,at 1150℃,Zn existed in the ceramsite as oxide.The volatilization of heavy metals in low Si/Al sludge was inhibited by adding Na₂S at 700-1150℃.The volatilization rates of Zn and As increased by 2.3%and 0.5%,Pb and Sb decreased by 3.1%and 20.1%at1150℃after adding Na₂S in high Si/Al sludge.XRD analysis showed that Zn Ga O₄and Zn Cr O₄oxides are formed at 1150℃.Therefore,at low temperature,heavy metals might produce sulfates that were difficult to volatilize and inhibited the volatilization of heavy metals.With the increased of temperature,the heavy metal sulfate was converted into oxide and solidified in the ceramsite.The volatilization rates of Zn,Pb,As and Sb decreased by 38.6%,6.97%,72.8%and 60.8%at 1150℃when Ca O was added to low Si/Al sludge.The volatilization rates of Zn,As and Sb decreased by 6.5%,16.8%and 29.1%,and Pb increased by 3.4%at 1150℃after Ca O was added to high Si/Al sludge.XRD analysis showed that the stable Ca Pb₃Al₂Si₁₀O₂₄（OH）₆was formed when Pb reacted with Ca O at 700℃and 800℃,which inhibited the volatilization of Pb.In addition,Ca O had adsorption effect on heavy metals.With the increased of temperature,ceramsite appeared melting phenomenon,so that more heavy metals were wrapped in the glass phase material,so the inhibition effect on the volatilization of heavy metals was obvious.