| Incineration is playing an increasingly important role in municipal solid waste(MSW)management with the increasing of MSW production.Fly ash,the product of solid waste incineration(MSWI),needs to be treated before being transported to sanitary landfills.Fly ash disposal technology of China is relatively backward,such as cement solidification and chemical stabilization.There is a growing trend that hydrothermal processes will become a promising technology for fly ash disposal because it offers considerable merits in terms of economic,technical,and environmental effectiveness.Therefore,in this research,instead of conventional heating,microwave heating for hydrothermal treatment was used to stabilize the heavy metals of MSWI fly ash.This research was funded by national basic research program of China(No 2011CB201500),and conducted a series of fundamental experiments and theoretic studies in the issue of microwave-assited hydrothermal treatment.First,characteristics of fly ash samples were studied to compare with the treated fly ash.Compared with coal fly ash,the contents of Si and Al in MSWI fly ash are much lower.The major heavy metals present in the fly ash samples are Zn,Cu,Pb,and Ba,and their presence is related to the incineration of garbage..Three kinds of CFB fly ash and three kinds of grate fly ash are carried out by microwave-assisted hydrothermal stability research of heavy metals,and the main influence factors and mechanism of stabilization are further addressed.This study demonstrated that microwave-assisted hydrothermal treatment is a feasible method for the solidification of heavy metals in MSWI fly ash(both CFB and grate fly ash)and the leaching contents of hewavy metals in treated samples are detected below the specified limits set by GB16889-2008.To analyse the morphological changes of heavy metals in fly ash,sequential extraction procedure(SEP)recommended by European Community Buteau of Reference(BCR)is applied and the pH-dependence leaching testand Visual MINTEQ simulation are conducted.It is revealed that the proportion of acid-soluble and reducible fractions decrease,meanwhile the proportion of oxidizable and residual fractions increase in treated samples,which is consistent with the leaching experimental results.Based on the morphological changes of heavy metals,it is concluded that microwave-assisted hydrothermal process exhibits a high potential on heavy metals curing and decrease the total soluble content.Based on microwave-assisted hydrothermal stability research of heavy metals in MSWI fly ash,the degradation of dioxins by this treatment is studied,including the influence factors and the analysis of degradation path.It is showed that dechlorination and destruction are supposed to be the main pathways for the decrease in PCDD/Fs(or other chlorinated aromatics)in fly ash,with the effect of microwave-assisted hydrothermal process.Under the microwave condition,dioxin degradation is efficient rapid and it is found that the chlorination reaction evidently exists during the hydrothermal process.The additive and water-washing pretreatment will sharply weaken the chlorination reaction and accelerate the degradation of PCDD/Fs.To provide a basic experimental foundation for the application of recycling of MSWI fly ash,the adsoption characteristics of raw and treated fly ash are compared.The results indicate that the cation adsorption capacity of treated fly ash is about 0.5 meq/g.Compared to the raw MSWI fly ash,the adsorption capacity is increased about 21 times.In all,microwave-assisted hydrothermal treatment can be applied to the simultaneous disposal of dioxins and heavy metals in fly ash.Under the action of microwave,the disposal efficiency is greatly improved,also the disposal cost is sharply reduced.Thus microwave-assisted hydrothermal treatment has a strong industrial application prospects. |
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