| Municipal solid waste incineration (MSWI) fly ash is classified as a special hazardous waste world-widely for the presence of leachable heavy metals, high concentrations of soluble salts. These hazardous substances in fly ash can pollute groundwater and soils, some of them are difficult to degrade and could accumulate, thus they are harmful to organisms when exposes to organisms. Thus, fly ash must be detoxified or decontaminated prior to disposal or reuse. At present, various approaches have been used to solidify/stabilize fly ash, such as cement fixation, acid extraction, stabilization with chemical agents and vitrification. From the long term view of environmental security, these heavy metals remained in fly ash will not only threaten the environment, but also cause waste of metal resources.Some heavy metals, such as Pb, Cu, Zn, Cd will evaporate during the heat-treatment, then formed solid again during cooling process. On the basis of these,"Thermal separation process (TSP)"was presented firstly and"Second fly ash(SFA)"was defined in this paper. During thermal separation, heavy metal will evaporate and be enriched in SFA. This method not only has solved environment security problem but also recoveried metals source.Optimization of TSP parameters determined that temperature and time were the main influence factors with the optimum values 1000℃and 120min, respectively. The evaporation rate of Pb can reach as high as 99.7% under the optimum conditions,and the Cd, Cu, Zn were 89.7%, 77.9% and 53.2%, respectively. If temperature and time are further enhanced, the evaporation of Cd and Cu can reach 98% and 95%, and Zn also achieve above 90%.The addition of different types of chlorides such as CaCl2,MgCl2,AlCl3,FeCl3 and NaCl, as well as"wash-ash"method were adopted to investigate the chlorination mechanism. Two theories, equilibrium partial pressure of chlorides and Standard Gibbs Free Energy Change for reaction, were brought out to explain the chlorination mechanism. In order to test the mechanism artificially synthetic fly ash was used in the experiment , the result showed that the simulating results were in fine identity with the experimental data ,which verified the mechanism validity.Thermodynamic Model and Kinetic Model were established based on the results of TG-DTG analysis. Friedman and the Ozawa law were used to calculate"Energy"and the pre exponential factor"A". By three-step reaction mechanisms, A --(Fn)-> B --(Fn)--> C --(Fn)--> D, the thermodynamic Model of fly ash was simulated asv=kcn=Ae-E/RTcn. On the basis of chlorination mechanism, Kinetic Model for heavy metal thermal separation was proposed as C=Cg(1-e-kt/h)。Physico - chemical analysis indicated that the dry SFA was a kind of filemot powder, it would change gradually from filemot to azure after moisture absorption, with the moisture further increased, the azure powder would become cyan or aqua slurry in the end. The result of SEM demonstrated that shape and size of particles in SFA varied differently, mostly in the form of irregular block and clava. Lots of different shape small particles were clinged to big granules. Compared with the original fly ash, the content of heavy metals in SFA raised greatly. The mass percentage of Pb was 10%, which was 5 times of original one, the content of Zn and Cu were 8.7% and 5.7%, respectively. The SFA was mainly composed of Cl, Pb, Cu, Zn, K, accounted for the total quantity 98.1%, and Cl accounted for about 40%. Therefore, SFA can be reused as metallurgy materials or enriched into metal ore. Analysis of X-ray diffraction(XRD) indicated the SFA was mainly composed of metal chlorides, such as PbCl2,KPb2Cl5, et al. The result further confirmed that the heavy metal were evaporized with chlorides during the heat-treatment.In conclusion, the result of experiment shows that the method of TSP is feasible to treat the fly ash. It can solve the environment potential hazard problem which was created by the tradition methods for fly ash treatment. |
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