Zeolite Fabrication From MSWI Fly Ash By Modified Hydrothermal Methods,and The Applications

Municipal solid waste incineration(MSWI)fly ash is a hazardous waste with considerable yield.Hydrothermal method is considered as an effective technology for the harmless disposal of MSWI fly ash.Besides,MSWI fly ash contains silicon and aluminum elements,which can be used as raw materials for synthesizing zeolite during hydrothermal process in theory,improving the application potential.But,the researches on zeolite synthesis using MSWI fly ash are few,and quality of zeolite synthesized by MSWI fly ash is not good.This is mainly because the crystalline quartz in MSWI fly ash is not easy to dissolve in the hydrothermal process.Thus,only a small number of silicon-containing ions are available in the hydrothermal solution,leading to low utilization of silicon and aluminum in MSWI fly ash.How to improve the utilization of silicon and aluminum in MSWI fly ash,as well as the quantity and quality of the produced zeolite should be explored.To improve zeolite synthesis using MSWI fly ash as the raw material,this research improved the utilization efficiency of silicon and aluminum components in MSWI fly ash by adding silicon-based reagents,changing the heating method(replace traditional conduction heating with microwave heating)of hydrothermal process,conducting fusion pretreatment.Thus,zeolite materials,such as Na P1 zeolite,were obtained.In addition,the porous zeolite structure of the hydrothermal products possessed catalytic and adsorption ability,which could be applied in photocatalytic degradation of dye,and adsorption of carbon dioxide and heavy metal ions.The modified hydrothermal process proposed in this paper could convert MSWI fly ash to zeolitic materials with low toxicity and high application potential.For the problem of low utilization of silicon and aluminum,soluble sodium metasilicate was added during the traditional hydrothermal process to improve the concentration of silicon ions,promote the reaction of silicon and aluminum elements,and increase zeolite content in the product.The effects of hydrothermal temperature,hydrothermal time,and the amount of sodium metasilicate on the composition,morphology,surface functional groups,thermogravimetric properties and cation exchange capacity of the products were studied.The heavy metal stabilization and the dioxin degradation in traditional hydrothermal products were investigated.On the basis of the results,in the traditional hydrothermal process,quartz was difficult to dissolve into hydrothermal solution.The problem could be effectively improved by adding a proper amount of soluble silicon-containing reagent,and zeolite Na P1 was successfully synthesized.For the traditional hydrothermal temperature,Na P1 zeolite was synthesized at 120?and 150?,and was converted to analcime when the temperature was increased to 180?.The cation exchange capacity of the product at 150?was 1.85meq/g,which was the highest.For the traditional hydrothermal time,it took 9 h or more to successfully synthesize Na P1 zeolite.Plenty of time was required for the dissolution of active ingredients and the nucleation and crystallization of zeolites.On the basis of adding silicon-based reagents,the traditional conduction heating was replaced by the microwave heating,so that the silicon and aluminum materials dissolved and reacted more quickly.The effects of microwave-assisted hydrothermal temperature,microwave-assisted hydrothermal time,the dose of sodium metasilicate and solvent were studied.The heavy metal stabilization and the dioxin degradation in microwave-assisted hydrothermal products were investigated.According to the studies,Zeolite Na P1 could be formed in only 1 h by microwave-assisted hydrothermal method,which was 88.9%shorter than the traditional hydrothermal method.When the microwave-assisted hydrothermal time was extended to 2 h,the properties of the product did not change significantly,indicating that the hydrothermal system reached equilibrium after 1 h of microwave-assisted hydrothermal treatment.As for microwave-assisted hydrothermal temperature,zeolite Na P1 could be synthesized when the temperature was raised to 180?,while zeolite Na P1 and analcime were formed in the product at 190?.When 1.02 mmol Si O₃^2-per gram of raw ash was added,hydroxy-sodalite was formed in the microwave-assisted hydrothermal product;When the amount of Si O₃^2-was increased to 1.02 mmol per gram of raw ash,zeolite Na P1 was formed;When the amount of Si O₃^2-was increased to 3.07 mmol per gram of raw ash or more,zeolite Na P1 and analcime were both formed.The introduction of Si O₃^2-increased the concentration of silicon element in the hydrothermal solution,so that the produced zeolite had a relatively high silicon content with more Si O₃^2-added.As for the amount of solvent(H₂O),when the amount of solvent was increased from 5 m L to10 m L or 20 m L,the zeolite composition of the solid products was always Na P1 zeolite.The solvent amount had no significant effect on the composition of the products.The dissolution of crystalline quartz during hydrothermal process is difficult,leading to the low utilization efficiency of silicon.Alkali fusion pretreatment was applied to MSWI fly ash.With fusion pretreatment,crystalline quartz transformed to amorphous silicon,which was easy to dissolve during hydrothermal process.In the absence of silicon-based reagents,many tobermorite and sodalite were obtained in the fusion-pretreated microwave-assisted hydrothermal product.The influence of fusion pretreatment on the subsequent hydrothermal process and products was investigated.Moreover,the strengthening effects of fusion pretreatment on the heavy metal stabilization and dioxin degradation,as well as the effect of fusion atmosphere were studied.The results showed that the fusion pretreatment could transform the quartz in the MSWI fly ash into amorphous and silicate,which was easy to dissolve in the subsequent microwave-assisted hydrothermal process,promoting the synthesis of tobermorite and sodalite.In terms of the performance of the product,the CEC of fusion-pretreated microwave-assisted hydrothermal product was 1.172 meq/g,which was more than 2 times of that of the direct microwave-assisted hydrothermal product without fusion pretreatment(0.428 meq/g),and was nearly 50 times of that of the raw ash(0.023 meq/g).The heavy metal leaching concentration of fusion-pretreated microwave-assisted hydrothermal product was obviously reduced.The dioxin degradation efficiency of fusion-pretreated process was more than 99.9%.The change of fusion atmosphere from oxidizing atmosphere to inert atmosphere had no obvious effect on the composition of the product,but it could inhibit the oxidation of Cr and reduce the migration and leaching of Cr.The applications of hydrothermal products from MSWI fly ash were verified,including copper ion adsorption,photocatalytic degradation of methylene blue,and carbon dioxide adsorption and separation on the laboratory scale.For 500 mg/L copper ion solution,the adsorption capacity of hydrothermal product was about 61 mg/g,and the adsorption rate was about 61%.The fly ash hydrothermal product not only had large specific surface area,but also contained metal components with catalytic activity,which could be used to catalyze the degradation of methylene blue.The removal rate of methylene blue could reach 97.6%after 24 h of continuous photocatalytic degradation under 1 k W/m² simulated sunlight.The hydrothermal product was applied to the adsorption of carbon dioxide,and the adsorption capacity of the hydrothermal product for CO₂ at 60 bar was about 0.7 mmol/g,which was 5 times that of the raw ash.The hydrothermal product showed selective adsorption for the CO₂/N₂ system and could be used for separating carbon dioxide and nitrogen.It was proved in this paper that the modified hydrothermal process could transform MSWI fly ash to valuable zeolite and reduce the harms of the heavy metals and dioxins in MSWI fly ash,benefitting the environment and waste circulation.