Study On Alkaline Hydrothermal Treatment For Stabilization Of Heavy Metals In Fly Ash From Municipal Solid Waste Incineration

In recent years,the quantity of municipal solid waste incineration(MSWI)has increased rapidly in China,and MSWI is becoming the practical ways of the treatment of municipal solid waste.But MSWI fly ash from incineration process were classified as a hazardous waste due to contaminants enriched in it such as heavy metals and“Three Cause” organic pollutants(dioxins,PCBs,PAHs).Therefore,fly ash must be treated harmlessly.In this context,the hydrothermal process was applied to deal with heavy metals in fly ash.Experimental studies of the addition of exogenous silica-alumina source additives and the recycling of alkaline hydrothermal liquid,the addition of phosphate on the effect of the stabilization of heavy metals in fly ash.The objectives of the research as follows:(1)Aming at the low contents of Si,Al and unsuitable proportion of the elements in fly ash,study on the screening of silica-alumina source additives and the stabilization of heavy metals by hydrothermal treatment.The results indicated that bottom ash and added coal fly ash have hydrothermal synthesised analcime and tobermorite.They were both low-cost and high efficient.It is conducive to the hydrothermal synthesis of tobermorite that bottom ash and coal fly ash were used as silica-alumina source additives.Tobermorite which stabilized heavy metals were hydrothermal synthesized by single addition of 30% coal fly ash and mixed addition of 15% coal fly ash+15%diatomite,mixed addition of 20% coal fly ash+30% bottom ash,due to regulating elements proportion to Ca/(Si+Al)=0.67~1.20.The leaching concentration of Pb、Zn、Cu、Cd、Cr in hydrothermal products by single and mixed addition were decreased by84~97%、86~97%、44~82%、52~95%、50~97% which were far below the hazardous waste identification standard.By using acetic acid buffer solution,the leaching concentration of Pb after hydrothermal treatment was decreased by 67~86%.In addition,the concentration of Pb,Zn in hydrothermal liquid phase by single and mixed addition of silica-alumina source additive above respectively decreased obviously.(2)The stabilization of heavy metals in fly ash was studied with the recycling of hydrothermal liquid and the results indicated that tobermorite can’t be synthesised in three-stage hydrothermal process by single addition of coal fly ash with 100% recycling of hydrothermal liquid.As the amounts of silica-alumina source additives increases,the leaching concentration of heavy metals in hydrothermal products and the concentration of heavy metals in hydrothermal liquid decreased obviously.Under the multistage 100%、75%、50% recycling of hydrothermal liquid,tobermorite were synthesized by mixed addition of coal fly ash and bottom ash,and the leaching concentration of heavy metals in hydrothermal products and the concentration of heavy metals in hydrothermal liquid were both lower under 50% recycling of hydrothermal liquid,especially Pb.Under 50%recycling of hydrothermal liquid with different alkaline activator,tobermorite can’t be synthesized with Na2CO3 by mixed addition of coal fly ash and diatomite,but can be synthesized with NaOH.The leaching concentration of heavy metals in hydrothermal products and the concentration of heavy metals in hydrothermal liquid phase both decreased.In addition,the contents of Pb,Cr in multistage hydrothermal products increased along with the increasing of recycling obviously,suggesting that the transfer of heavy metals to liquid phase can be inhibited by the recycling of hydrothermal liquid.(3)By changing types and contents of phosphate,liquid-solid ratio,temperature under hydrothermal conditions,the curing rates of the leaching concentration of heavy metals in raw fly ash compared to treated fly ash were studied by using orthogonal designs.The results indicated that the effect of stabilization of heavy metals were better with Na3PO4 under hydrothermal conditions.The optimized parameters of stabilization of heavy metals with phosphate were: 1)phosphate is Na3PO4,additive contents is0.1mol/kg,liquid-solid ratio is 3mL/g,temperature is 175℃;2)phosphate is Na3PO4,additive contents is 0.7mol/kg,liquid-solid ratio is 3mL/g,temperature is 150 ℃.In addition,after the optimum treatment,the leaching concentration of Pb decreased to1.61 mg/L,1.42 mg/L,respectively.The results of Pb were lower than that of orthogonal test,the leaching concentration of Zn,Cu,Ni were decreased obviously.Via the study of this paper,Under alkaline hydrothermal treatment of fly ash,accurate regulation of Ca、Si、Al key elements in hydrothermal system and efficient hydrothermal synthesis of aluminosilicate minerals,co-treatmrnt of fly ash and bottom ash were realized by the screening of addition technology of silica-alumina source additives which were both low-cost and high efficient.Under elements regulation,the multistage recycling of hydrothermal liquid and additional phosphate assisted hydrothermal process have improved the stabilization of heavy metals,decreased the concentration of heavy metals in hydrothermal liquid,reduced emissions of waste water,provided theoretical and technical supports for further realization of pollution of hydrothermal treatment of fly ash to "zero emission".