| Recently, with the advantages of harmlessness, mass and volume reduction as well as nery recovery, Municipal Solid Waste Incineration(MSWI),has becoming an increasing dominant strategy of solid waste disposal in China.However, the fly ash derived from the incineration process has been seriously classified as hazardous merial due to high quantities of soluble salts, significant amounts of toxic heavy metals in forms that may easily leach out and trace quantities of very toxic organic compounds (dioxin, furans) in most countries. Therefore, MSWI fly sh must be adequately treated bfore landfill or recycling as a secondary material. In an attempt to dispose the fly ash safely and eco-efficiencily, the research funded by national basic research programme of China(973 programme,2011CB201500),was conducted incluing treatment of water-washing method and on-fired brick made for secondary material. The main objectives and results are involed as follows.1) Two types of fly ash were choosen from the air pollution control devices (APCDs) two mainstream domestic incineration technology including stoker grate (SG) and circulating fluidized bed (CFB), and the characteristics of them were analysed and compared. The results showed that Zn, Cu and Pb accounted for the majority of the total amounts of heavy metals in fly ash. the leachate of CFB fly ash contained high concentration of Cd(1.948mg/l)、Cu(60.29mg/l)、 Pb (5.36mg/l), far beyond the limit of Chinese sanitary disposal standard. While in SB fly as only the leaching toxicity of Pb(0.42 mg/l)and Cd(0.81 mg/l)slightly exceeded the standard value.In both fly ashes, PCDF contributed to the most of I-TEQ of dioxin.The pH of leachate has significant influence on leaching behavior of heavy metals in fly ash.The leaching concentration almost reached the maximum when pH<3, decreased with the increasing of pH from 3-11 reaching almost negligible at the pH of of 7～11 except Cr which exhibit a narrow rebound and after pH> 11, a sharp increase of leaching of Pb and Zn. MINTEQ modeling analysis showed that he impact of pH on metals leaching behavior was realized by altering the chemical forms of heavy metals in leaching sytem.2) For the two types washing experiments were systematicly conducted to investigte the leaching behavior of chloride salts and heavy metals during water-washing process and the framework of nearly zero emissions of the wastewater derived from washing process was proposed. The results showed that chloride in fly ash mainly exsist in NaCl, KCl, CaCl2, CaClOH form, and washing is extremely effective in the removel of chlorine. The removel rate can reach to up 92%and73% for GB fly ash and CFB fly ash, respectively. The majority of the component in the washing leachate is soluble chlorides, accounting more than 97%, and Mg, Al, Fe and heavy metals exhibit extremely minor leaching. Optimal washing conditions for SG fly ash are that liquid to solid ratio is 6, the washing time is 10min, the oscillation frequency was 200rpm and secondary wateing is unnecessary. While for CFB fly ash the optimal conditions involes first washing with liquid to solid ratio of 4, washing time of 30min and the oscillation frequency of 200rpm, and secondary water with liquid to solid ratio of 4, washing time of 30min, the oscillation frequency of 250rpm. pH is an important factor in controlling chemical speciation distribution of heavy metals (Pb, Cu, Zn, Cd) during washing process while Cl- also has an important influence on the chemical speciation of Cd. Washing process little effcet on the concentration and I-TEQ fingerprint distribution of the dioxin in fly ash. Regression analyses indicated electrical conductivity (EC) values in leachate has a strongly linear correlation with concentration of Cl-, thus making it possible to achieve on-line monitoring and process parameter optimization via EC value.3) Based on the above results, CFB fly ash was typically utilized to make non-burned bricks as secondary material using cement and chelating agents, and the dominant performance was studied. The modified sequential extraction procedure by Community Buteau of refrerence (BCR) was adopted to analyse the speciation distribution of heavy metals in fly ash and brikes, and then environmental pollution risk and bio-availability of heavy metals are evaluated by univariate risk assessment code (RAC) and Nemerow omprehensive pollution index. The results showed that cement exhibited a sound efficiency in immobilizing Cu、 Zn、 Pb in fly ash, but insignificant effect on Cr. The bricks with 30% cement could reduce the leaching toxicity far below the standard limit. The compressive strength of the matrix improved as the percentage of cement loading was increased. The compressive strength was 12.8MPa for 30% cement and met the Chinese national standard building requirement for MU15 class for 35% cement. Compared with the untreated fly ash, the leaching behavior of heavy metals in matrix during different pH was not changed, but the leaching concentration have remarkably declined, and acid neutralizing capacity and the safe PH region have been improved. The acid extractable and exchangeable fraction of Cd、Zn、Cu in untreated fly ash reached 48%、21.26%、20.72%,resectivly,and the Nemerow comprehensive pollution index was on an extremely high level of 38.23.While the acid extractable/exchangeable fraction of heavy metal in matrix with 30% cement had reduced far below 1%, RAC decreased above two orders of magnitudes, and the Nemerowomprehensive pollution index declined to 0.022. These were explained by the fact that acid extractable/exchangeable fraction was mostly changed to reducible and oxidational fraction during the solidification of cement. The stability of the chelating agentfor heavy metals has strong selectivity. Trimercapto-s-triazine (TMT) and Sodium Dithiocarbamate (DTCR) exhibited an effective stabilization of Cu, Pb, Cd and Cu, Pb, respectively, and the immobilization effect of TMT was better than DTCR with the same dosage. The dosages of the two cheating agents to meet the standard of the landfilling standard were 2% and 3%. In the process of co-stabilization, the relationship between the cement addition and agent dosage was not linear when satisfying the landfilling requirement. The chelating agent could avoid the obvious compatibilization, it posed a remarkable increase of treatment cost, though. |
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