Research On Process Mechanism And Fundamental Application Of Hydrothermal Activation Of Fly Ash

Fly ash,a bulk industrial solid waste,is one of the main solid wastes discharged by coal-fired power plants.At present,fly ash is mainly used in the building material industry,which belongs to a kind of low value-added utilization.The high value-added utilization of fly ash mainly includes the extraction of conventional elements Si and Al to prepare white carbon black and alumina,the synthesis of zeolite molecular sieve and the extraction of precious metal elements.The efficient activation of fly ash is necessary for its high value utilization.The traditional high-efficiency activation method includes the alkali fusion method.Fly ash and solid alkali are mixed evenly and then roasted at high temperature to perform the activation.This method has high alkali consumption and energy consumption,whose post-processing steps are complicated.Therefore,this paper proposes a green,efficient,convenient and recyclable activation method of fly ash,choosing pulverized coal furnace fly ash and circulating fluidized bed fly ash obtained from two typical combustion methods to explore their differences in the hydrothermal process.Based on this,the widely used and high value-added molecular sieve synthesized from two kinds of fly ash are studied.The paper mainly studies the mechanism of hydrothermal activation conditions and the physicochemical properties of fly ash on the activation products;analyzes the transformation process of silicon aluminum in fly ash(mainly exists in the amorphous glass and crystalline phase of quartz and mullite)to the activated sample(low silica-alumina ratio zeolite)in detail;studies the migration and conversion of alkali metals,alkaline earth metals and transition metals in fly ash and their influence on activation products during the hydrothermal activation;analyzes the migration law of Si,Al,Ca,Na,Mg,Fe,Ti and other elements in the activated sample combined with the acid leaching method;and reveals the process mechanism of hydrothermal activation of fly ash combined with XRD,ICP-OES,SEM and other characterization.The results show that under high temperature and high alkali conditions(temperature is higher than 300?,NaOH solution concentration is greater than0.6 mol/L),pulverized coal furnace fly ash are hydrothermally activated,and Si and Al in fly ash are activated quickly and transformed into octahedral zeolite crystals with flower-shaped microcrystalline clusters.Approximately 45%of Si element in fly ash is dissolved in the alkaline solution during the heating process of activation,and other elements are present in the solid phase.As the reaction proceeds,the Si in the solution participates in the growth of zeolite crystals,then combines with active Al in the solid phase to form the octahedral zeolite.The pulverized coal furnace fly ash is activated under milder experimental conditions(temperature is lower than 300?,NaOH solution concentration is less than 0.5 mol/L).The result shows that the activation effect of the fly ash has increased sensitivity to the activation temperature and changes in alkali solution.After pulverized coal furnace fly ash is hydrothermally activated under this condition,it is mainly composed of amorphous silicoaluminate on the surface layer and converted into crystalline zeolite.The mullite in the center of the pulverized coal furnace fly ash microspheres still exists in the activated sample(Analcime).The activation process of the pulverized coal furnace fly ash is carried out layer by layer from the surface layer to the inner later of fly ash.The hydrothermal activation process of pulverized coal furnace fly ash can be described as:at the initial stage of the reaction,NaOH in the alkaline solution diffuses to the outer surface of the fly ash,and the Si element in the fly ash dissolves in the alkaline solution;as the reaction proceeds,NaOH diffuses from the outer surface to the inner surface of fly ash,and reacts with the amorphous aluminosilicate on the surface of fly ash to activate the Si and Al therein,and the Si in the solution starts to combine with the activated Si and Al in the solid phase of the fly ash into a zeolite framework.When the activation temperature is 200? and NaOH solution concentration is 1.0 mol/L,the initial reaction is controlled by internal diffusion and the late reaction is controlled by chemical reaction.The reaction rate constants are 4.8×10^-4 and 3.0×10^-3,respectively.Thus,the activation of fly ash is mainly controlled by the internal diffusion of the initial reaction.The combustion temperature of circulating fluidized bed is low,and the minerals in the coal are not melted to form a dense and smooth glass body during the combustion process.The morphology of fly ash is mostly irregular.At the same time,coal is usually mixed with Ca-based desulfurizing agent,resulting in the high free CaO content in circulating fluidized bed fly ash.All these promote the activation and transformation of Si and Al in fly ash,which can be fully activated under relatively milder conditions(temperature is lower than 300?,NaOH solution concentration is less than 0.5 mol/L).By analyzing the difference between pulverized coal furnace fly ash and circulating fluidized bed fly ash,the fly ash-based zeolite with a low silicon-aluminum ratio is obtained by hydrothermally activating pulverized coal furnace fly ash.The widely used CHA type SSZ-13 molecular sieve is synthesized combined with the hydrothermal conversion method.The circulating fluidized bed fly ash and iron-containing waste(red mud and steel slag)are hydrothermally activated in one step to synthesize the recyclable fly ash-based magnetic zeolite,which is used to remove elemental mercury(Hg⁰)from coal-fired flue gas.The results show that the Hg⁰ removal efficiency of magnetic zeolite modified by HCl is almost 100%at 200?,and the efficiency is over 80%after the first regeneration.In addition,the magnetic zeolite modified by HCl has a wide active temperature window,which make it more competitive in practical applications.