Characteristics Of MSWI Fly Ash During Pressure-assisted Sintering Process At Low Temperature

Municipal solid waste incineration fly ash was classified as a hazardous material due to the high content of leachable heavy metals and dioxins,therefore,the disposal of fly ash has gained great public concern these years.Sintering treatment was effective for solid waste disposal due to the advantages of volume and weight reduction and value-added materials production.Due to the existence of chloride,it was difficult to form viscose flow,leading to the high sintering temperature(1000℃).And the high content of chloride in fly ash would be evaporated into the atmosphere in the form of heavy metal chlorides.In addition,the watersoluble chlorides would be sublimated and recrystallized in the decarburization zone,resulting in the blockages and corrosion process.To stabilize heavy metals and improve mechanical properties of sintered samples simultaneously,the chemical compounds transition and thermo-dynamics of fly ash during thermal treatment was investigated firstly,then the physico-chemical properties and heavy metals leaching behavior were enhanced by pressure pretreatment and coal fly ash addition.And the pressure-assisted sintering process was introduced for fly ash disposal to enhance the heavy metals stabilization and mechanical properties at low sintering temperature,and the chemicals transition and mass transfer mechanism were investigated.In addition,the molecular dynamics simulation of pressure-assisted sintering process was carried out to explore the migration and diffusion process of fly ash.Firstly,the migration and transition of chemical compositions and heavy metals in fly ash was investigated via TG analysis and the reaction mechanism was determined by Málek method.The results showed the main crystal phases were transformed from NaCl,KCl,CaCO3,CaSO4 and CaClOH to Ca10(SiO4)3(SO4)3Cl2,K2Ca2Mg(SO4)4·2H2O,Mg3Ca(CO3)4,MgSiO3 and Ca6(Si2O7)(OH)6 during the thermal process.In addition,three stages were observed during the whole thermal process and could be expressed by F1,D3 and R3 models,respectively.Then investigation of fly ash sintering characteristics with pressure pretreatment and different chemical compositions was carried out.Coal fly ash was added for fly ash composition regulation.The results revealed that the compressive strength of sintered samples increased from 2.9±0.1 MPa to 6.4±0.3 MPa,which was attributed to the increase of crystalline degree.When the pretreatment pressure increased to 100 MPa,the compressive strength of sintered samples was increased to 51.0±0.9 MPa.This phenomenon was mainly due to the increased contact area between fly ash particles.In addition,the leaching concentration of heavy metals was lower than the standard limitation.To further decrease the sintering temperature,the pressure-assisted sintering was introduced for fly ash disposal.The results suggested that the compressive strength was increased from 13.7±2.1 MP to 218.3±4.1 MPa,which was 4.28 times to that of samples with coal fly ash addition and pressure pretreatment.In addition,the sintering process was affected by the diffusion process,the sintering kinetics results indicated that the mass transfer mechanism was transformed from surface diffusion at 300℃ and 400℃ to grain boundary diffusion at 500℃.These findings suggested that pressure-assisted sintering could improve the mechanical properties and enhance the mass transfer process of fly ash samples.Moreover,the migration and transition behavior of chloride in fly ash was incestigated.The results suggested that with the increase of temperature and pressure,the migration rate of chloride was increased,leading to the heterogeneous distribution on the surface,which could provide concentration gradient for chloride migration.During the pressure-assisted sintering process,the chloride was evaporated fractionally,and the residual chloride was transformed from NaCl,KCl and CaClOH into Ca10(Si2O7)2SiO4Cl2(OH)2 and Ca10(SiO4)3(SO4)3Cl2 minerals,resulting in the leaching concentration of chloride.To explore the enhancement mechanism of mechanical property and heavy metals leaching behavior in sintered samples,the pressure-assisted sintering process was carried out for fly ash with different chemical compositions.The results indicated that compressive strength of sintered samples was increased from 92.2±7.1 MPa to 238.3±8.5 MPa when the molar ratio changed from 0.5 to 1.0,which was due to the formation ofAl2SiO5,Ca11(SiO4)4O2S,Ca3Si2O7 and Ca3Al2SiO7.The formed minerals linked and contacted with each other to form stable frameworks,which positively affected the compressive strength.And increased applied pressure enhanced the densification process.At last,the micro-aggregate vibration theory was introduced for the pressure-assisted sintering process of fly ash and verified by molecular dynamics simulation.The results showed that the application of mechanical pressure increased the migration rate of atoms and enhanced the contact between atoms as well,leading to the porosity decreasing.Therefore,the molecular dynamics simulation provided a new sight for the exploration of pressure-assisted sintering mechanism.In summary,the pressure-assisted sintering for fly ash disposal could decrease the sintering temperature obviously,and it was feasible and effective to improve the mechanical property and decrease the heavy metals leaching risk.In this process,the densification and the mineral transformation was accelerated.These findings proved that pressure-assisted sintering could be a promising method to treat fly ash together with producing high-value materials.