Basic Research Of The Preparation Of Mullite From High Alumina Fly Ash By Synergistic Activation Method

High alumina fly ash（HAFA）is mainly generated in the midwestern Inner Mongolia/north of Shanxi and Ningxia region,whose emission amount is about 30 million tons annually.But the comprehensive utilization ratio is lower,and its accumulation leads to serious pollutions.Simultaneously,abundant alumina and silica resources are contained in HAFA,whose contents reach to 45%and 35%respectively,and the mullite/corundum/amorphous phase are the main mineral phases.In terms of the characteristics of elements and mineral phases,the preparation of mullite is the strategic demand for the comprehensive utilization of HAFA,and the elevation of Al/Si ratio by mild desilication is the key point.In this work,because of the low Al/Si ratio and high impurities contents in HAFA,some mild activation processes have been investigated,and the preparation of mullite by mechanical-chemical activation-deep desilication process has been promoted.The activated mechanism of different mild activation/mechanism of silica sol to gel during the acid activation/efficient separation between amorphous phases and crystal phases/preparation of polyaluminum chloride by activated acid solution and preparation of mullite by desilicated HAFA have been studied in detail.The main contents and conclusions are as follows:（1）In terms of the reactivity of amorphous silica in HAFA,the activation index（EDR）is promoted to estimate the reactivity of amorphous silica.Firstly,the effects of mechanical activation/chemical activation and microwave activation on EDR are researched respectively,and the chemical activation can elevate its EDR obviously;simultaneously, the mechanical-chemical activation/microwave-chemical activation and mechanical-microwave activation on EDR have also been studied,and the mechanical-chemical activation is the best method.Secondly,the mechanical-chemical activation process is optimized,and the EDR can reach to 12%under the condition of mechanical process（t=90min）and acid activation process（T=75-80℃,t=90min,C=6mol/L,L/S=4:1）,at the same time,the contents of Fe/Ca/Mg impurities have also been decreased to below 1%.Lastly,the mechanism of synergistic activation has been analyzed,the analysis by EPMA/BET and NMR indicates that the bond Al-O-Si in amorphous phases is complicated,after the synergistic activation,the small particles are distributed,and the encapsulated amorphous and crystal phases are destructed.The inert Q4（3A1）structure in the amorphous phase is transferred to active Q4（2A1）and Q2（1A1）,which promotes the decompose the Al-O-Si bonds by H+,and the specific surface area is elevated to 2 times,which increase the active sites of Si-O-.Therefore,the impurities are removed,and the reactivity of amorphous silica is improved obviously.（2）In terms of the difficult separation between solid and liquid caused by the transformation from silica sol to gel during the acid activation,based on the theories of DLVO and Zeta potential,a novel method to analyze the transformation from silica sol to gel by zeta potential/viscosity and pH online continuously is established.The morphology of colloidal particles and changes of Si-O-Si bonds during the potential mutation period have also been analyzed and verified by the in-situ FT-IR and TEM,which presents that the silica sol is transferring to silica gel during the potential mutation period.Therefore,the effects of different valent cations（K+,Na+,Ca2+,Mg2+,Fe3+,Al3+）on the silica sol to gel are studied,which indicates that the higher concentrations and valences of cations accelerate the gelation process,and this process can be alleviated when the pH is controlled under 1.90 and the concentrations of cations is below a lower level;simultaneously,the effects of different anions（Cl-,Br-,I-,N03-,SO42-,PO43-）on the silica sol to gel are studied,which presents that the aggregation of silica sol is accelerated when the ionic radius is decreased and the valence is increased,which can destroy the stable electric double layers.（3）In order to accomplish the deep desilication and avoid the side reaction,firstly,based on the results of orthogonal experiments,the effects of different factors on the desilicated ratio and mineral phase are investigated,and the optimal results indicate that the desilicated ratio can reach to 60%under the condition:T=95℃,t=90min,C=6mol/L,L/S=5:1,simultaneously,the contents of Na can be controlled below 0.5%.Secondly,the changes of complicated Al-O-Si bonds/occurrence state of elements and phases/changes of pores in HAFA are investigated,which indicates that the amorphous Si-O bonds are decomposed by alkali solution.Lastly,the kinetics of deep desilication are studied,and the results indicate that the earlier stage is mainly controlled by surface reaction,and the later stage is mainly controlled by solid film diffusion due to the zeolites formed on the surface of particles.（4）In terms of the waste acid solution generated by acid activation process,a novel method to prepare the polyaluminum chloride is promoted.The concentration of Al/Fe/Ca ions are elevated from 3.0g/L,2.5g/L,2.5g/L to 22.0g/L,14.0g/L,10.0g/L respectively when the recycling times of acid solutions is 8 under the optimal conditions:T=85℃,t=90min,ratio between calcium aluminate and H+=0.3-0.5,and the Al₂O₃ contents,basicity and density in polyaluminum chloride reach to 11%,76%and 1.1g/mL respectively,which can meet the industrial demand well.In terms of the desilicated HAFA,the mullite grains grow to columnar mullite crystal under the conditions:moisture content=8%,no additives,forming pressure=168MPa,calcination temperature=1650℃,calcination time=2-3h,and the Al₂O₃ contents,bulk density and apparent porosity reach to 70%,2.85g/cm3 and below 0.5%.Besides,the material balance,material flow analysis and preliminary economic evaluation have been finished to support the fundamental data for engineering application.