Solidification Of Toxic Heavy Metals In Municipal Solid Waste Incineration (MSWI) Fly Ash With Melting Blast Furnace Slag And Safe Disposal Of Its Reconstructed Slag

Municipal solid waste incineration (MSWI) fly ash is classified as a specialhazardous waste world-widely for the presence of leachable heavy metals dioxin andhigh concentrations of soluble salts. Thus, MSWI fly ash must be detoxified ordecontaminated prior to disposal or reuse. At present, various traditional curingtechnologies have been used to solidify/stabilize fly ash, such as cement solidification,bitumen solidification, stabilization with chemical agents, sintering and meltingsolidification, however, from the view of disposition effect, disposal costs and longterm environmental security, the effect is not ideal. The heavy metals remained in flyash will not only threaten the environment, bring about secondary pollution, but alsocause resource waste.MSWI fly ash has low activity and the residual dioxin and toxic heavy metals arepotential danger, which make it difficult to be used in the field of building materialsfor a large amount of safety consumptions. To resolve these technical difficulties andkey scientific problems, MSWI fly ash-blended molten slag at high temperaturefollowed by reconstructing with water quenching, and the reconstructed slag was usedas a cement replacement in hydration process to achieve the dual solidification oftoxic heavy metals. The main results are as follows:(1) MSWI fly ash-blended molten slag at high temperature and its heavy metalssolidification in this process were studied. The reconstructed slag present glassylong-range disorder. XRD, SEM and IR analysis results show that there construction,chemical composition and mineral composition of reconstructed slag are similar to theundisturbed slag. The29Si NMR results showed that the two polymeric peaks ofQ1(-77.1ppm) and Q4(-118.0ppm) of the reconstructed slag containing ultra-riskMSWI fly ash became significantly broaden and weaken compared to the two peaksof the original slag. And the transfer of Q0to Q1means that the polymerization degreeincreased and the activity decreased. The results of the TCLP tests for thereconstructed slag showed that the concentrations of leached heavy metals were below the regulatory threshold values, which realized the first solidification goal of heavymetals in ultra-risk MSWI fly ash, when slag-replacement percentage of ultra-riskMSWI fly ash was30%. In particular, the volatilization rate of Pb is high at hightemperature; only a small portion in the form of aluminosilicate is effectivelydissolved in reticular substrate through the physical encapsulated adsorption, thevolatile heavy metals such as Pb and Zn, which partition into flue gas in the form ofbimetallic chlorides, such as Na2ZnCl4·3H2O, K2ZnCl4and KPb2Cl5during themelting process. What is more, the substitution of Zn, Cu, Pb and Cr with Ca2+andAl3+of silicate and subsequent dissolution in the reticular basal body during themelting process, Si-O reticular structure was formed, then heavy metals transferred tothe molten slag were encapsulated and solidified in the dense grid of glassed materialduring the melting process. Thereby, the heavy metal leaching concentrations of thereconstructed slag were reduced significantly. However, the concentrations of leachedheavy metals increased or even exceed the regulatory threshold values with slagreplacement percentage of ultra-risk MSWI fly ash more than50%.(2) The performance of cement-blended reconstructed slag and the dualsolidification of heavy metals were studied. In this study, the melting andsolidification of MSWI fly ash with the heat carrier, molten blast furnace slag, isfirstly proposed. After melting and water quenching, the reconstructed slag is obtainedand recycled as a cement replacement in cement specimens, achieving dualsolidification of heavy metals. Heavy metals solidification in the III process weredivided into two parts: one was the free heavy metals which had no effectivesolidification during the Ⅱprocess, in the early stage of composite cement hydration,the free heavy metals reacted with hydration products and consolidated in thecomposite cement system. The other was the effective solidification part of free heavymetals in Ⅱprocess, in the process of cement hydration, its products and structurechanged, then the heavy metals would release due to loss protection. And the releasedheavy metals would be consolidated by the cement hydration and return to thestability condition.(3) The safety of dual solidification system in severe erosion environment wasanalyzed. The whole process was divided into4main stages: incubation period, expansion period, expansion maintaining period and disintegrating period. Afterdifferent destructive cycles,4typical heavy metals’ leaching concentration ofspecimens were still significantly lower than the maximum permitted concentration ofnational standards.The conventional performance and stability of heavy metal in severe erosionenvironment were studied, the safety threshold of reconstructed slag#8is50%. Basedon this threshold, the actual consumption of ultra-risk MSWI fly ash in the cementcomposite system was calculated to be25%at least (regardless of the ignition loss ofultra-risk MSWI fly ash during melting reconstruction process). The dual-curingtechnology of MSWI fly ash can make full use of waste heat of blast furnace slag,having a good curing effect and low cost. This technology can achieve the synchronyof processing and using of MSWI fly ash, solve the contradictory problem that theconventional technologies still cannot successfully manage to balance the disposaleffectiveness and costs of MSWI fly ash so far, and realize the objective ofeconomizing energy and reusing waste resources，having high practical value andbroad applicative prospects.