Study On The Integration Of Municipal Solid Waste Incineration Fly Ash Carbonation And Stabilization Of Heavy Metals

Incineration,as a new technology for disposal of municipal solid waste,has been widely used to handle municipal solid waste in recent years,since the presence of high levels of heavy metals in fly ashes after incineration,the municipal solid waste incineration?MSWI?fly ash has been classified as hazardous waste and must be handled effectively and safely before being landfilled or reused.In addition,greenhouse effect caused by excessive carbon dioxide emissions has great effect to human environment and has been received a global attention.In the treatment technology of carbonation the CaO and metal oxide in MSWI fly ash could react with CO2 and form the most stable carbonate,at the same time,the carbonation not only improve stabilization of fly ash but also reduce greenhouse gas emissions,thereby achieving the goal of“waste control by waste”and owning a good application prospect.By combining with the designed experiments and theoretical analysis,a systematic research on MSWI fly ash carbonation was performed by studying the relationship between the physicochemical characteristics and leaching characteristics of heavy metals;then the influence factor on carbonation and heavy metals,including reaction temperature,time,pressure,mass ratio of liquid to solid,atmosphere,etc.,was explored and key transformation point was elaborated;furthermore the role of competitive inhibition and synergic promotion on oxy-fuel combustion flue gas in MSWI fly ash carbonation was researched in particular;finally the reaction mechanism of carbonation was revealed by establishing the dynamics models.Firstly,a detailed research and contrastive analysis concerning the physical and chemical compositions of the six types of MSWI fly ash samples was performed,achieving the relation between leaching characteristics of heavy metals and physical properties of MSWI fly ashes.It's beneficial and instructive to develop corresponding control measures and treatment methods for different types of MSWI fly ash through laboratory studies.The results showed that the major elements in the six kinds of MSWI fly ash are Si,Ca and Cl and the main crystal compositions are CaO,CaClOH,NaCl,Ca?OH?₂,KCl and SiO₂.It's indicated that the leaching concentrations of heavy metals were gradually increased with the decrease of the particle size of fly ash,and the pH has a very great impact on the leaching characteristics of heavy metals:under neutral,weak acid and alkalescence conditions the leaching concentrations of Pb,Zn and Cu were lowest,but extremely increased under strong acidic and strong alkaline conditions;the leaching concentrations of As,Cr,Co,Cd,Ni and Hg were lowest under neutral and alkaline conditions,but significantly increased with the decrease of the pH value.Subsequently,the performance of different reaction conditions on carbonation experiment were performed.The results showed that the physicochemical properties of fly ash were changed significantly by the carbonation reaction.After the carbonation reaction,it's clear that the increased content of CaCO₃ and the disappearance of CaO,Ca?OH?₂ and CaClOH in original fly ash.With an increase of reaction time,the carbonation conversion in all three samples increased and eventually tended to be constant.With an increase of reaction temperature,the carbonation conversion increased and reached a maximum at approximately100?and then slightly decreased with increasing the reaction temperature.With an increase of reaction pressure,the carbonation conversion increased quickly and the optimum reaction pressure was about 1 MPa.Continuing increasing the reaction pressure,the carbonation conversion was almost unchanged.With an increase of mass ratio of liquid to solid,the carbonation conversion reached a maximum when the mass ratio of liquid to solid was 0.3.The results of carbonation reaction in different atmosphere show that the carbonation conversion reached a maximum with pure CO₂ and decreased sharply with low content of CO₂ in reaction atmosphere.The carbonation conversion would be further lower with a certain amount of SO₂ in reaction atmosphere.Then the leaching experiments on the carbonated fly ash samples were performed and we observed that the leaching concentrations of heavy metals were obviously decreased after carbonation.With an increase of reaction time,the leaching concentrations of heavy metals in all three kinds of samples decreased and eventually tended to be constant after the reaction time of 60 min.With an increase of reaction temperature,the leaching concentrations of heavy metals decreased and reached a minimum at approximately 100?.Continuing increasing the reaction temperature,the leaching concentrations of heavy metals slightly increased.With an increase in the reaction pressure,leaching concentrations of heavy metals decreased quickly.When the reaction pressure is greater than 1 MPa,the leaching concentrations of heavy metals were almost stable.With an increase in the mass ratio of liquid to solid,the leaching concentrations of heavy metals decreased firstly and then increased.The leaching concentrations of heavy metals reached a minimum when the mass ratio of liquid to solid was 0.3.In carbonation reaction the leaching concentrations of heavy metals were related to the reaction atmosphere composition and the content of CO₂ in it.It's indicated that the carbonation conversion reached a maximum with pure CO₂ and the leaching concentrations of heavy metals with pure CO₂ was further lower than that with 15%content of CO₂.The leaching concentrations of heavy metals were higher with a certain amount of SO₂ in reaction atmosphere.Therefore,in this paper the reaction time of 60 min,the reaction temperature of 100?,the reaction pressure of 1 MPa,the mass ratio of liquid to solid of 0.3 and the pure CO₂ is the optimum conditions,in which the leaching concentration of Pb in Changzhou sample decreased from 31.54 to 1.4 mg/L,meeting standards of landfill.Additionally,the carbonation reaction with simulated oxy-fuel combustion flue gas was emphatically studied and compared with pure CO₂ and simulated air combustion flue gas.The results showed that the reaction atmosphere had great effect on the carbonation conversion and stabilization of MSWI fly ash.Compared with air combustion flue gas,simulated oxy-fuel combustion flue gas had higher carbonation conversion and lower the leaching concentrations of heavy metals,which were closer with pure CO₂.Although the acid gases in oxy-fuel combustion flue gas go against the carbonation reaction,the effect can be neglected due to the low concentration of acid gases and the high concentration of CO₂.Therefore,carbonation of MSWI fly ash with oxy-fuel combustion flue gas could be a low-cost and promising technique for the harmless treatment of MSWI fly ash.Finally,the carbonation reaction mechanism of MSWI fly ash was studied and a two-layer diffusion kinetic model,which be named as product layer and residue layer,was formulated.The model showed a better description on carbonation of MSWI fly ash by distinguishing the relationship of product layer and residue layer.It's concluded that the results achieved from the model accurately matched the experimental data of carbonation reaction of MSWI fly ash:the rate of the carbonation reaction in the initial phase was faster and gradually decreased as the reaction proceeded until to the end,which was consistent with the experimental results.During the carbonation reaction process,the product layer became thickness gradually,which influenced the ion exchange,at the same time,the core layer was gradually consumed and the residue layer was thickening,all the thing above was the reason.