Separation and Utilization of Silica from Alumina Extraction Process of Coal Fly Ash with Acid Leaching

Author:Liu Dan Dan

Supervisor:cheng fang qin zhang zuo


Degree Year:2019





Coal fly ash(CFA)is a major solid waste of coal-fired power plant,whose annual emissions exceed 500 million tons.The comprehensive utilization efficiency is low,and its accumulation causes serious environmental problems.CFA contains more than 30% Al2O3 and 50% SiO2.The development of feasible alumina extraction technology has become an urgent strategic requirement for the utilization of CFA.The extraction efficiency of alumina reaches 7090% with acid-leached process.The silica was converted into highly active amorphous phase,avoiding secondary pollution.It is the key to improve the mass ratio of Al2O3/SiO2 and separation efficiency of aluminiumsilicon in the process.In this paper,in view of the transformation,separation of silica derived from alumina extraction process,the mechanisms of predesilicating and separation were investigated.For the utilization of silica resources,the routes of “one-step” preparation of white carbon black and “nontemplate” synthesis of mesoporous silica using pre-desilication liquids and Alextracted residues were developed.It will provide technical support for the high-value utilization of SiO2 derived from alumina extraction process of CFA.The main contents are as follows:(1)The combination of acid-leaching pretreatment and EDTA complexation was carried out to intensify pre-desilication.The effects of Na OH concentration,reaction temperature and addition amounts of EDTA on the desilication efficiency were investigated.The transformation of mineral phase and enhanced mechanism were also studied.The results showed that the amorphous phases were removed in the pre-desilication process,including amorphous silica and alumina.The reduction of alumina dissolution and the complexation of aluminium hindered the formation of hydroxyl sodalite.The pre-desilication efficiency was improved from 38% to 58%.(2)The separation conditions were studied and optimized using BoxBehnken experimental design method.The results showed that the method can effectively predict the influence of HCl concentration,reaction temperature and reaction time on the separation efficiency of Al-extracted residues.The HCl concentration and reaction temperature were significant factors.The content of SiO2 in Al-extracted residues reached 85% under optimized conditions.Meanwhile,the effects of surfactants on the separation efficiency of Al-extracted residues were studied.The content of SiO2 in Al-extracted residues reached 92% due to the dipole-ion interaction between CTAB and PEG.(3)The white carbon black was prepared with assistance of surfactants CTAB and PEG using Al-extracted residues.The particle size,DBP absorption value and specific surface area of white carbon black were 3.20 μm,3.16 cm3/g and 404 m2/g under optimized conditions.Considering the instability of CFA,the influence mechanism of calcium and iron impurities on the preparation of white carbon black was investigated.It was found that Ca2Fe2O5 appeared with increasing the contents of Fe2O3 and Ca O.The method could be used to prepare white carbon black when the contents of Ca O and Fe2O3 in CFA were less than 6% and 15%,respectively.Lastly,the dispersity of white carbon black was improved by the steric hindrance of mercapto-silane coupling agent.The particle size was 557 nm,and the absolute of Zeta potential in ethanol reached 36.43 m V.(4)The mesoporous silica was prepared by non-template sol-gel using pre-desilication liquids and Al-extracted residues.The effect of sodium silicate properties and synthesis conditions on the pore structure properties were studied.The mesoporous silica with specific surface area of 690 m2/g was obtained under optimized conditions.The pore structure of mesoporous silica mainly controlled by the polymerization reaction rate and degree of silicate colloid.Furthermore,the adsorption performance for Pb2+ of mesoporous silica was investigated.The adsorption process conforms to Langmuir isotherm model with a maximum adsorption capacity of 330 mg/g.The adsorption rate for Pb2+ was controlled by membrane diffusion resistance and intraparticle diffusion resistance.