| With the popularity of waste to energy power generation in China,the production of municipal solid waste incineration fly ash(MSWI FA)has been increasing annually.MSWI FA is rich in heavy metals,dioxins and other harmful substances,causing secondary pollution to the ecological environment.Stabilization/solidification(S/S)through cement is the most widely applied method in China,but the traditional cement method has high carbon emissions and low addition of MSWI FA.Thus,adhering to the concept of "treating waste with waste",coal gangue(CG)and blast furnace slag(BFS)were used as solidifying materials for the treatment of MSWI FA.A series of experimental and theoretical studies on S/S of heavy metals were carried out,as well as preliminary exploration on the industrial application of these methods.The major contents and conclusions are as follows:(1)Ten MSWI FA samples from grate-type(GT)and circulating fluidized bed(CFB)incinerators were collected and compared in terms of physicochemical properties,heavy metal leaching toxicity,and dioxin toxicity.The results are as follows: the design and combustion conditions of these two incinerators have great influence on the basic physical and chemical properties of MSWI FA.The particles shape and size of the GT fly ash are uniform,while those of CFB fly ash are irregular.The main components are chlorides in the GT fly ash and oxides in the CFB fly ash.The excessive heavy metals in the GT fly ash are mainly Cd and Pb,while those in the CFB fly ash are mainly Cd,Ni,Pb and Se.The I-TEQ concentrations of dioxin in the GT fly ash ranged from 0.308 to 0.546 ng I-TEQ/g,while those in the CFB fly ash ranged from 0.015 to 3.312 ng I-TEQ/g.(2)The leaching characteristics of heavy metals in MSWI FA under simulated acid rain corrosion and landfill leachate corrosion were compared.The results are as follows: landfill leachate corrosion is much more severe than acid rain corrosion,and the leaching rate of heavy metals under landfill leachate corrosion is higher,so the long-term leaching risk will be higher.Under simulated landfill leachate corrosion conditions,the long-term leaching behavior of heavy metals Cd,Cr,Cu,Ni,Pb and Zn all follow the "two-stage pattern",that is,the rapid release and accumulation in the first stage,and the cumulative leaching rate in the second stage decreases and gradually reaches a peak.For the GT fly ash,Cd,Pb and Zn are the most accumulated heavy metals,while Cr,Cu and Ni are the most accumulated heavy metals in the CFB fly ash.(3)CG is used to prepare geopolymer and solidify MSWI FA.Through optimization,a method of using CG and calcium oxide to treat fly ash is developed.The results are as follows: the use of CG solely to solidify fly ash has poor S/S efficiency of heavy metals.The fly ash addition is only 20%,in order to meet landfill requirements.The activation treatment of CG has no obvious improvement effect on the compressive strength and the S/S of heavy metals.Adjusting the ratios of sodium hydroxide-sodium silicate composite activator,high temperature curing,and adding active calcium oxide can promote the development of mechanical strength of solidified bodies and the S/S efficiency of heavy metals.When the fly ash content is 60%,the optimum addition amount of calcium oxide is10% to obtain the highest compressive strength of solidified bodies.In order to make heavy metal leaching toxicity meet landfill standards,the minimum addition amount of calcium oxide is 20%,and the leaching concentrations of Cd and Pb are 0.06 mg/L and 0.02 mg/L,respectively.The addition of calcium oxide promotes the formation of CSH,CASH gel and zeolites(such as hydrocalumite,calcite and sodalite)in the polymerization reaction and hydration reaction.These products stabilize the heavy metal ions in fly ash through physical encapsulation,surface adsorption and ion exchange.Meanwhile,the cooperation of calcium oxide facillitate the evolution of Cd,Pb,Cu and Zn from soluble and oxidizing acid fractions to reducible and residual fractions.The solidification scheme of CG-calcium oxide also has excellent effect on the S/S of heavy metals in fly ash from different cities and incinerators.Compared with the traditional cement disposal method,the solidification scheme of CGcalcium oxide has the advantages of high fly ash addition(60%)and volume reduction(-27.13%).(4)BFS was used to prepare geopolymer to solidify fly ash,and sodium diethyldithiocarbamatre(DTC)was added to improve the S/S efficiency of heavy metals.The effects of different DTC addition on the S/S of heavy metals and mechanical strength of solidified bodies were studied.The results are as follows: the stabilization efficiency of Cd and Pb is low when using BFS to treat fly ash solely.In order to meet landfill requirements,the addition of fly ash should not exceed 30%.With the increase of DTC addition,the compressive strength of fly ash-BFS solidified body decreases,but the S/S efficiency of heavy metals is enhanced.The optimal addition of DTC is 1%,and the S/S efficiency of Cd and Pb can reach more than 99%.The S/S effect by BFS is due to hydration and geopolymerization products.Excessive addition of DTC would hinder hydration reaction,and the improvement of S/S of heavy metals by the addition of DTC resulted from the chelation reaction between DTC and heavy metals.The solidification scheme of DTC-BFS also has excellent effect on the S/S of fly ash from different cities and incinerators.The solidification scheme of DTC-BFS has the advantages of higher fly ash addition(60%)and volume reduction(-19.12%).(5)A pilot test device for solidifying MSWI FA was designed and established,with a maximum daily processing capacity of up to 1.5 tons/day.The high S/S performance of these two solidification schemes was verified on the device,of which the operation cost was reducted compared with the traditional cement disposal method.The basic technical scheme of the industrial application is designed.The solidification process and system are put forward,and the technical parameters and requirements of industrial application are standardized. |
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