Synthesis Of Mesoporous Silica Using Alumina-extracted Residue Derived From Coal Fly Ash

Coal fly ash（CFA）is the main solid waste generated from coal-fired power plants.Its annual production reaches about 600 million tons,but the utilization is only⁷0%,which causes serious environmental hazards to the land,water and air.Coal fly ash contains large amounts of alumina and alumina extraction from it becomes one of the important value-added utilization ways.More than 90%of alumina can be extracted and the silica can be converted to highly active amorphous phase via―Pre-desilication–Na₂CO₃ activation–Hydrochloric acid leaching‖process,which achieves the integrative utilization of Al₂O₃ and SiO₂ resources in CFA.Besides,due to the increased molar ratio of Al/Si of fly ash by the pre-siliconization process,the amount of sodium carbonate used is reduced by about 20 to 50%.In this paper,the alumina-extracted residue（Al-R）produced in the process was dissolved in desilicated solution（De-SS）to prepare high modulus sodium silicate（HSS）,and the HSS was used as the raw material to synthesize mesoporous molecular sieve SBA-15 for the adsorption of heavy metal ions.The main contents and the results are as follows.（1）Using different silica sources as raw materials,the dissolution properties of Al-R,quartz sand and commercially available silica in the low modulus sodium silicate solution（LSS）were studied comparatively.The reaction conditions of the dissolution properties of Al-R in LSS were optimized.Fourier transform infrared spectrometer（FT-IR）and X-ray diffraction（XRD）were used to analyze the chemical bond and phase of silica sources.The results showed that the dissolution of commercially available silica and Al-R containing the high contents of active amorphous silica can reach more than 95%,but the quartz sand is hardly soluble in SS at<100 ^oC.The optimum conditions for the reaction between Al-R and SS are100 ^oC for 30 minutes,which can increase the modulus of the SS to more than 3.（2）The effect of the impurities on the SiO₂ extraction of Al-R,the modulus and compositions of sodium solution（SS）was studied.X-ray diffraction（XRD）and inductively coupled plasma emission spectrometry（ICP-OES）were used to analyze the phase and chemical composition of Al-R and SS.The results showed that the alumina in Al-R would react with sodium silicate solution to form precipitation and therefore decreases the dissolution of Al-R.When the content of Al₂O₃ is 0.1%or less,the dissolution of Al-R is not affected,and the modulus of the De-SS can be increased to>3.The impurities such as Al₂O₃,Fe₂O₃,CaO and TiO₂ in Al-R are almost insoluble in the De-SS.Therefore,the high modulus SS prepared by the Al-R can meet the industrial standards.（3）Effect of the different modulus SS prepared from De-SS and Al-R on the preparation of SBA-15 was studied.The effect of the molar ratio of SiO₂to template agent P123（SiO₂/P123）on the order and structure of SBA-15was investigated.HSS made from fly ash and pure SS were used as raw materials,the structural differences of the prepared SBA-15（denoted as FSBA-15 and SBA-15 respectively）were compared.The SBA-15 modified with 3-aminopropyltriethoxysilane（APTES）was used to the adsorption of heavy metal ions.The structure of the material was analyzed by X-ray diffractometer（XRD）,physical adsorption instrument and transmission electron microscope（TEM）.The results showed that the order of the mesoporous SBA-15 synthesized was higher with the modulus of SS increasing.Meanwhile,the specific surface area,pore volume and pore size increased but the micropore surface area decreased.When SiO₂/P123 was 40,the order of the material is low,and with the SiO₂/P123 increased to 58 and80 the order increased greatly.With the increases of SiO₂/P123,the specific surface area and pore volume decreased,the surface area of the micropore increased.The order and structural parameters of FSBA-15 synthesized from coal fly ash and SBA-15 from pure-SS were similar.Both FSBA-15 and SBA-15 were regular hexagonal structure.The impurities such as Al₂O₃,Fe₂O₃,CaO,and TiO₂ in FSBA-15 were less than 0.2%.FSBA-15 and SBA-15 were modified with amino group and used to adsorb heavy metal ions.There were few differences in the adsorption heavy metals between FSBA-15 and SBA-15.The adsorption of Cr（Ⅵ）,Pb（Ⅱ）and Cu（Ⅱ）reached more than 90%,respectively.Cr（VI）adsorption process of NH₂-FSBA-15and NH₂-SBA-15 were consistent with Langmuir isotherm at 25°C.