Heavy Metals Removing From Municipal Solid Waste Incineration Fly Ash By Electric Field Enhanced Washing

Municipal solid waste incineration?MSWI?fly ash is a kind of highly polluting hazardous waste which accounts for 3%¹5%of the solid waste incineration.By reason of its high content of soluble salts,heavy metals and persistent organic pollutants with high toxic,MSWI fly ash need to harmless disposal before landfill or resource utilization.Therefore,the safe and efficient disposal of pollutants in MSWI fly ash has become one of the hot spot of academic and social.In this paper,the physico-chemical property and pollution characteristics of MSWI fly ash in beijing during whole year were identified.Base on the theory of chemical process intensification and electrochemistry,the research was carried out on the removal of heavy metals from MSWI fly ash by electric field.The significant results were obtained as following:1.The results of physico-chemical property shown that the dominating elements in MSWI fly ash are Ca,Cl,Si,S,Na,and K,and mainly in the form of KCl,NaCl,CaSO₄,CaClOH,CaCO₃,SiO₂ and Ca?OH?₂.MSWI fly ash mainly in the shape of spherical and multi-horny,no complete crystal basically.The metals with the highest average content in MSWI fly ash were Zn,Pb and Ba,was 3488.55 mg/kg,760.15 mg/kg and 616.09 mg/kg.And the content of Zn,Pb,Ba,Cu and As in MSWI fly ash shown higher in summer and autumn than in spring and winter.In addition,the leaching toxicity of Pb in MSWI fly ash exceeded the threshold value of standard?GB5085.3-2007?.The average annual bioavailability concentration of Pb in MSWI fly ash was 49.32 mg/kg,and Ba was 30.49mg/kg.The third is Zn,with an average value of 24.92 mg/kg,the average concentration of Cd was 11.66 mg/kg.The leaching concentrations of metals in MSWI fly ash in acidic environment?pH=2?reached the maximum,which Zn was 12.84 mg/L,Pb was 5.49 mg/L,Ba was 2.25 mg/L,Cu was 0.34 mg/L,and Cr was 0.17 mg/L.Cd and Zn in MSWI fly ash had higher leaching risk,which soluble&exchangeable states reached 30.47%and 28.97%.Pb and Cu are relatively stable,and the sum of reducible and oxidizable state exceeds 65%.The residual states of Cr,Ni and As were 77.52%,58.24%and 30.99%,respectively.2.The experiment of heavy metals removing from MSWI fly ash by electric field-enhanced washing showed that under the optimal parameters of current intensity was35 mA/cm²,polarity switching frequency was 40 Hz,EDTA dosage of 0.5 mol/L,and a pH of 2 for 4 h,Cd and Ni are almost completely removed from MSWI fly ash and the removal efficiency of Pb,Cu,Zn and As were 97.78%,94.26%,80.19%and 74.69%,respectively.Through an applied electric field and polarity switching frequency could changed the surface charges distribution of MSWI fly ash,accelerated the electron transfer in solution,and alleviated the passivation of electrodes,improved the removal efficiency of heavy metals.Under certain conditions,the removal efficiency of Ni,As,Pb and Cd were increased by23.37%,31.27%,10.91%and 2.57%,when compared with no applied electric field.3.Based on the applied electric field,the effects of different assistant agents on the removal of heavy metals from MSWI fly ash were studied.The experimental results are as follows:the removal efficiency of Pb,Cd and Ni treated with citric acid was better,with the removal efficiency reached above 90%.And the removal efficiency of Cu,Zn and As reached 72.68%,83.12%and 71.86%,respectively.When ammonium citrate was used,the removal efficiency of Pb,Cu,Zn,Cd,Ni and As in fly ash were 69.07%,88.18%,77.94%,91.51%,86.43%and 69.57%,respectively.The material consumption ranking list of assistant agents are EDTA>ammonium citrate>citric acid,and considering the economic efficiency and effective of treatment,citric acid has better performance.