Synthesis,Growth Mechanism And Adsorption Performance Of Zeolites Based On Coal Fly Ash

Coal occupies the main position of energy consumption in China,however,the huge emissions of coal fly ash（CFA）as by-product possess great threat to global environment and human health.Therefore,it’s becoming the most essential point to dispose and utilize CFA.Although the utilization of CFA is extensive,it is mainly limited to the low-end field.Thus,it is of great significance to recycle CFA with high efficiency and added value,reducing its negative impact on the environment.Indeed,the synthesis of zeolites from CFA can broaden the ways of resource utilization of CFA with economic and environmental benefits effectively.However,the growth mechanism of zeolites synthesized with CFA needs further study due to the complexity at present,and zeolites synthesized with CFA still display some technical shortcomings such as low purity and crystallinity,poor controllability of crystal form,and so on.In addition,the treatment of water and gas pollution has become an urgent problem as the rapid development of industry and social economy,nowdays,the development of low-cost and efficient adsorption materials is an important part of pollution control.Therefore,researches are carried out based on the two hot issues of CFA resource utilization and environmental pollution control in this paper.Different types of CFA based zeolites are synthesized through distinct crafts,as a result,the growth mechanism of zeolites was revealed,and the structure-activity relationship between characteristics of CFA based zeolites and characteristics of pollutants was established.On this basis,the adsorption mechanism of pollutants on CFA based zeolite was clarified,which provides a theoretical basis for the efficient utilization of CFA in order to exploit cheap and high-performance adsorption materials.The main results obtained in this paper are as follows.（1）The researches on optimization of zeolite structures by CFA pretreatment.The influence of acid leaching conditions on the dissolution law of impurities in CFA was investigated.The results showed that the impurities removal efficiency was high,Fe₂O₃and Ca O contents were decreased from 8.97%and 10.36%to 5.11%and 2.41%respectively under the conditions of hydrochloric acid concentration of 5.0%,liquid-solid ratio of 20,acid leaching temperature of 85℃,and acid leaching time of 2 h.And the contents of other impurities were also reduced.The experimental results of different activation methods showed that alkali fusion activated method could completely convert the inert components of CFA into soluble substances with an improved ultilization rates of CFA,while the inert components could not be utilized completely by alkaline solution activation method.The synthesized conditions based on alkaline solution activation method were optimized and zeolite P possessed properties of relative crystallinity of 80%,specific surface area of 55.9 m²·g^-1,and total pore volume of0.0933 cm³·g^-1 was obtained.The relationship between preparation parameters and zeolite structure was sysmetically studied,which was based on alkali fusion activation method.The significant influence of Si/Al ratio in precursor solutions on the crystal form of zeolites was also observed.The composite zeolite A-X was synthesized when only CFA was used as raw material to provide both silicon and aluminum sources.The results showed that zeolite A was preferentially formed at lower temperature and shorter time,and the increase of Na OH content was beneficial to the formation of zeolite A,while the increase of calcination temperature,hydrothermal temperature and time was conducive to the synthesis of zeolite X,which was conducive to the improvement of specific surface area.Meanwhile,both cubic and octahedral morphologies representing zeolite A and zeolite X respectively were observed,with an average particle size of 3.26μm.The zeolite A possessed physical properties of average particle size of 1.53μm,relative crystallinity of 95%,specific surface area of 43.7 m²·g^-1,and total pore volume of 0.0568 cm³·g^-1was obtained under milder conditions when Si/Al ratio was adjusted to about 1 by the introduction of external aluminum source.（2）Studies on the synthesis of CFA based small crystal-size zeolite.Both zeolites X and Y with small crystal-size were synthesized by seed assisted hydrothermal method without adding any template or organic additives,in which the optimized conditions with seed crystal of 18Na₂O:Al₂O₃:14Si O₂:370H₂O and anging time of 4 d were ensured.The crystallization rate of zeolite could be accelerated greatly and the formation of miscellaneous crystals could be inhibited simultaneously by the addition of seed crystal,which was induced by the four-membered ring active unit of Si/Al=3.With the addition of optimized seed crystal,CFA based small crystal-size zeolite X with single crystal phase was obtained as only CFA was used as raw matierial,while single crystal and CFA based small crystal-size zeolite Y was synthesized with both CFA ad external silicon source.Both zeolites possessed high thermal stability,accompanied with the average particle size decreased from 1.49μm to 0.17μm of zeolite X and 0.12μm of zeolite Y,respectively.（3）Researches on the growth mechanism of CFA based zeolite.The evolution mechanism of framework structures was explored through the analysis of atomic connection mode and surrounding coordination state during the synthesis process of zeolite.The results revealed that a large number of four membered rings emerged in gel during the initial formation stage,but the compositions of these rings were different.Zeolite A was consisted of four-membered rings with Si/Al=1,while zeolite X and Y possessed four-membered rings with both Si/Al=3 and Si/Al=1,showing unique point lied in the different proportion of secondary structural units.The crystallization process could be expressed as following.For zeolite A:four-membered ring→βcage→αcage→zeolite A crystal;For faujasite zeolite:four-membered ring→six-membered ring→βcage→faujasite cage→faujasite zeolite crystal.（4）Studies on the adsorption performance of CFA based zeolites.The adsorption matching law between different kinds of CFA based zeolites and pollutants was clarified.The results showed that the best adsorption performance of CFA based zeolite A for ammonia nitrogen solution was observed,while CFA based zeolite X and Y showed better adsorption effect on toluene and benzene respectively.This was because the pore size and Si/Al ratio of zeolite as well as the spatial configuration,molecular size and polarity of adsorbate all affected the adsorption property.The pseudo-second order kinetic model could better reflect the adsorption behavior of ammonia nitrogen on CFA based zeolite A,while Bangham model was responsible for the description of toluene and benzene adsorption processes on zeolite X and Y.Weber-Morris model fitting showed that the adsorption process was controlled by internal and external diffusion simultaneously,and Langmuir model possessed a higher fitting degree to the adsorption equilibrium isotherm.In addition,the average free energy values of ammonia nitrogen,toluene and benzene adsorbed by CFA based zeolite A,X and Y were 25.24 k J·mol^-1,6.940 k J·mol^-1 and 7.955 k J·mol^-1respectively calculated by D-R model.Based on the inference of above models,the adsorption of ammonia nitrogen by CFA based zeolite A was mainly induced by ion exchange,while the adsorption of toluene and benzene on CFA based zeolite X and Y were mainly physical adsorption.