Study On Stabilization/Solidification Of Mswifa By Geopolymericzation Based On Secondary Alumina Dross

Municipal solid waste incineration fly ash（MSWIFA）is classified as hazardous waste due to characteristics of high alkalinity,high salinity and high leaching concentrations of heavy metals.And secondary aluminum dross（SAD）generated during industrial production of aluminum is difficult in recycling.Consequently,a new method for stabilization/solidification of MSWIFA by geopolymerization based on SAD was proposed,which is of great significance for environment protection.The effects of addition ratio of alkali activator,calcination temperature,Si/Al molar ratio,and curing condition on stabilization/solidification of MSWIFA were investigated in this study.Tessier extraction procedure experiments o were carried out to analyze chemical changes of heavy metals.Crystal structures and microstructures of MSWIFA as well as solidified bodies were also analyzed by XRD and SEM to investigate stabilization/solidification mechanism of MSWIFA.The main conclusions of this paper are as follows:Reasonable alkali activator addition ratio and thermal activation temperature of SAD matrix can significantly improve stabilization/solidification effects of MSWIFA.With the increase of alkali activator addition ratio,leaching concentrations of heavy metal in the SAD-SiO₂-based solidified bodies with Si/Al molar ratio of 1.088 decreased significantly and compressive strength of the solidified bodies rose obviously.When addition ratio of alkali activator was more than 6%,leaching concentrations of Cr,Cd,Pb,Zn and Cu in the SAD-SiO₂-based solidified bodies decreased slowly and compressive strength of the solidified bodies was slightly increased.SAD matrix that is pretreated through calcination at certain temperature could enhance the activity of Al₂O₃ and SiO₂.The activation temperature range of SAD matrix was preliminary determined to be 600 to 800°C after leaching experiment of heavy metals and compressive strength test of solidified bodies.The results of XRD and SEM analysis showed that Ca₂（SiO₄）,Cancrisilite(formula is Na₇[Al₅Si₇O₂₄]CO₃·H₂O)and Ca₂Al₂SiO₆（OH）₂ were produced in solidified bodies obtained by geopolymerization,which could enhance compressive strength of solidified bodies and transform heavy metals from easily leached exchangeable fraction and carbonates fraction to stable residual fraction,organic matters fraction,and Fe-Mn oxides fraction,decreasing leaching concentrations of heavy metals.Si/Al molar ratio has a great influence on stabilization/solidification effects of MSWIFA.When molar ratio of Si/Al was less than 2.5,leaching concentrations of heavy metals in the SAD-SiO₂-based and Metakaolin-SiO₂-based solidified bodies decreased significantly and compressive strength increased obviously with the increase of Si/Al molar ratio.When molar ratio of Si/Al was greater than 2.5,leaching concentrations of heavy metals and compressive strength of solidified bodies tend to be stable with the increase of the Si/Al molar ratio.The results of XRD and SEM analysis showed that categories and contents of geopolymers in the Metakaolin-SiO₂-based solidified bodies were slightly higher than those in the SAD-SiO₂-based solidified bodies,and structure of the Metakaolin-SiO₂-based solidified bodies was denser than that of SAD-SiO₂-based solidified bodies,but stabilization/solidification effects of MSWIFA between two kinds of solidified bodies was small.As a result,secondary aluminum dross that Si/Al molar ratio was improved could be a potential substitute for metakaolin.Curing condition can significantly affect performance and structure of geopolymers and can also affect stabilization/solidification effect of MSWIFA.Under natural curing condition,stabilization time of heavy metals Cr,Cd,Pb,and Zn in the SAD-SiO₂-based solidified bodies with Si/Al molar ratio of 1.088 was mainly concentrated in the first 28 days,and stabilization time of heavy metals Cu was mainly concentrated in the first 14 days.Under curing condition of incubator,stabilization time of heavy metals in the SAD-SiO₂-based solidified bodies was reduced to be half that of the SAD-SiO₂-based solidified bodies cured under natural condition.Leaching concentrations of Cr,Cd,Pb,Zn,and Cu in the SAD-SiO₂-based solidified bodies decreased significantly and compressive strength of the SAD-SiO₂-based solidified bodies increased obviously with the increase of curing temperature in incubator.Crystals Na₇(Al₅Si₇O₂₄)（CO₃）（H₂O）and Ca₂Al₂SiO₆（OH）₂ grew obviously with the increase of curing temperature,which improved the stabilization/solidification effect of MSWIFA.When curing temperature was 50°C and curing time was 14 days,leaching concentrations of heavy metals Cr,Cd,Pb,Zn and Cu in the SAD-SiO₂-based solidified bodies decreased to 0.172,0.072,0.218,3.803 and 0.318 mg/L,which met the leaching requirements of"Standard for Pollution Control on the Landfill Site of Municipal Solid Waste"GB 16889-2008,and compressive strength of the SAD-SiO₂-based solidified bodies reached 8.315 MPa.Leaching concentrations of heavy metals in solidified bodies that cured at the temperature of 50°C decreased obviously and compressive strength raised much compared with solidified bodies cured at the temperature of 30°C.