| Up to now,the annual output of municipal solid waste (MSW) in China has been more than 170 million tons. Over two-thirds of Chinese cities have the difficulty to find the unused land to construct waste landfill and thus simply dumped solid waste has surrounded the cities. Incinerating MSW to produce electricity can not only solve the problem of land occupation by waste, but also achieve resource utilization of waste; and thus it has become an important alternative of waste treatment. Now, above 70 MSW incineration power plants has been established in China and they have the capacity to deal with about 25 million ton MSW per year. However, Fly ash coming from municipal solid waste incineration (MSWI) contains the high-toxicity compound dioxins and heavy metals. It is estimated that the annual output of MSWI fly ash is near to 1 million tons. These fly ashes seriously threaten the environment safety of our city. Therefore, it is very necessary to develop the technology for safe and efficient treatment of MSW incineration fly ash.Traditional disposal methods of MSW incineration fly ash, cement solidification, asphalt solidification, and chemical Stabilization, can not remove dioxin-like compounds. Melting and sintering can make MSW incineration fly ash to be completely harmless, but they need high cost and large energy consumption. Therefore, developing the technology for harmless treatment and resource utilization of MSW incineration fly ash, with the characteristics of energy saving and low cost, has a very important practical significance.On the basis of previous research results, this paper proposed a combined technology for resource utilizing MSW incineration fly ash: Use microwave heating to decompose dioxin-like compounds, then stabilize heavy metals using chemical stabilizer, and finally use the harmlessly-treated MSW incineration fly ash to make unbaked fly-ash ceramsite. This paper investigated the variation of dioxin degradation efficiency with the time and intensity of microwave irradiation at the range of temperature from 400℃to 1000℃. Meanwhile, the impacts of chemical stabilizers, such as diethyldithiocarbamate, potassium ethyl xanthate, ammonium dibutyl dithiphosphate, and Na2S, on heavy metal stabilization were compared. In addition, we studied the impact of the addition of portland cement and coal-fired power plant fly ash on the properties of unbaked fly-ash ceremsite, and cylindrical compress strength, bulk density, water absorption and other properties of unbaked fly-ash ceramsite were measured.The results showed that, the most optimum technological condition for dioxin degradation was to heat MSW incineration fly ash using microwave to 600℃and hold for 20 minutes. In this case, above 95% of dioxin in MSW incineration fly ash can be removed, and the dioxin can be reduced to below 100 I-TEQ ng/kg. The stabilization efficacy of heavy metals by thiol chelating agents was better than by other chemical stabilizers. when the application rate of thiol chelating agent was at 1×10-4mol/g, the ratio of stabilized heavy metals, Cu, Pb, Cd, Cr, Zn, and Ni, were more than 70%. The results of granulation experiment show that the most appropriate mass constitute of unbaked fly-ash ceramsite is: MSW incineration fly ash 50%～70%, portland cement 35%～45%, and coal-fired power plant fly ash 10%. the properties of the unbaked fly-ash ceramsite obtained by this method is: crushing strength 8.6～9.6MPa, bulk density 925～992kg/m3,and water absorption 8.2～8.9%. Furthermore, the production process of unbaked fly-ash ceramsite was optimized. |
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