Solidification/Stabilization Of Municipal Solid Waste Incineration Fly Ash And The Characterization Of Active Mineral Hydration Products

Municipal solid waste incineration fly ash is regarded as hazardous material, mainly due tocontaining high concentration of heavy metals. Solidification/stabilization (S/S) is an establishedutilization technology to treat municipal solid waste incineration fly ash. This work investigatedthe feasibility and effectiveness of stabilizing/solidifying MSWI fly ash with calcium aluminatecement, Portland cement, tricalcium aluminate, silica fume and metakaolin. The leaching behaviorof heavy metals, anion and organic matter were also studied in the fly ash and hydrated pastes.The compressive strength of the pastes curing28days from big to small in the order were:silica fume, metakaolin, aluminate cement, Portland cement and tricalcium aluminate. This agreeswith results of the average length of C–S–H gel chain.The structure and phase composition of fly ash and hydrated pastes were studied by XRD,DTA-TG and NMR analysis methods. The hydration products varied among different hydrationsystems. In calcium aluminate cement/fly ash system, hydration products were C–S–H gel,ettringite, monosulphate, Friedel’s salt, hydrated calcium aluminate and thaumasite. In Portlandcement/fly ash system, hydration products were C–S–H gel, ettringite, Friedel’s salt, Ca(OH)2andthaumasite. In tricalcium aluminate/fly ash system, hydration products were C–S–H gel, ettringite,monosulphate, Friedel’s salt and thaumasite. In silica fume/fly ash system, hydration productswere C–S–H gel, ettringite and thaumasite; in silica fume/fly ash system with high temperaturesteam, hydration product was C–S–H gel. In metakaolin/fly ash system, it was found that it ispossible to have geopolymeric gel and C–S–H gel forming simultaneously.In fly ash, the Pb concentration was exceeded the level of standard. The leachate pH washigher than the Chinese regulatory level (GB18598-2001). Comparison of OPC-20, CAC-20,SF-20, SF-W-20and MK-15in terms of Cu, Zn and Pb release, indicated that the effect ofsolidification/stabilization of MSWI fly ash by different materials were different. After curing for 7days the release of heavy metals were already low in the OPC-20and CAC-20. Between7daysand135days the leachate concentrations of heavy metals were almost constant in OPC-20whilethe leachate concentration of heavy metals gradually reduced in CAC-20. It seemed that SF-20exhibited the lowest heavy metal release after curing135days. High temperature steam speededup pozzolanic reaction, so heavy metal release from SF-W-20cured for31d were as low as theSF-20cured for135d. MK-15and TCA-20showed the worst effect of immobilizing heavymetals.As maturing of time and the percentage of active minerals in pastes increased, in distilledwater and sulphuric acid&nitric acid leaching tests, the leachate concentration of Pb, Zn and Cureduced. The leachate concentration of Cd was lower than the detected level. For the pastes,CAC-14and CAC-20cured for7d,28d and135d, SF-10, SF-14and SF-20cured for135d,SF-W-4, SF-W-6, SF-W-8, SF-W-10, SF-W-14and SF-W-20cured for31d, the Pb concentrationwere below the Chinese regulatory level5mg/L (GB5085.3-2007and GB18598-2001) andleachate pH were below12.0(GB18598-2001). So these pastes meet the Standard for PollutionControl on the Security Landfill Site for Hazardous Wastes (GB18598-2001).The pH dependent leaching characteristics were assessed for fly ash and hydrated pastescured for2years. The leaching curves of Cd, Pb, Cu and Zn shifted to lower pH value in differentdegrees, illustrating stabilization/immobilization of heavy metals in alkaline environment hadsignificantly effect. Geochemical speciation modelling by Visual MINTEQ was applied on theconcentrations of Al, Ca, Cr, Cd, Pb, Cu and Zn in the leachates in order to predict the measuredconcentrations and the specific release mechanisms. For the metal cations studied, the modelshowed that the formation of metal hydroxides in the alkaline pH region was not sufficient todescribe the leaching process for pastes, and that interactions with the hydrate phases were farmore important. Cr, Cd, Pb, Cu and Zn were controlled by the precipitation/dissolve and HFOadsorption mechanism. Al and Ca were controlled by the precipitation/dissolve mechanism.Combined the hydration products and leaching results of anion, Friedel’s salt, ettringite, andmonosulphate were synthesized to immobilize Cl-and SO2-4, and physical adsorption of Cl-on theC–S–H gel is also an important part. After curing for365days, the TOC and COD in leaching testwere reduced, while the toxicity of PAHs equivalent in pastes significantly reduced.