Synthesis, Characteri And Performance Of Layered Sodium Silicate From Acid-Extraction Residues Of Coal Fly Ash

Fly ash is the main solid waste generated by coal-fired power plants.The annual output is about 600 million tons in China.The accumulation amount is increasing year by year,and the utilization rate of fly ash is about 70%.The massive accumulation of fly ash not only occupies land,pollutes soil and water bodies,causes a series of environmental problems,but also endanger human health.The content of Al2O3 in fly ash is high,and extracting alumina from fly ash is one of the important ways for comprehensively utilizationt.At present,more than 90% of Al2O3 in fly ash can be dissolved in this process of extracting aluminum by fly ash acid method.In this process,a large amount of aluminum slag is generated,and how to effectively utilize aluminum residue becomes one of the urgent problems.In this paper,the preparation and properties of siliconbased materials by the desiliconization liquid and aluminum slag produced in two typical acid extraction processes were studied.Desiliconized liquid and aluminum residue in pulverized coal furnace(PC)fly ash pre-desiliconizationsodium carbonate activation-acid leaching process were used to prepare layered sodium silicate.Preparation of SBA-15 by aluminum residues produced in circulating fluidized bed(CFB)fly ash acid extracting process were performed,and the as-prepared SBA-15 is functionalized and applied to the adsorption of rare earth elements.The main results are as follows:(1)The layered sodium silicate is prepared by desiliconization liquid and an aluminum residue produced in the silicon ash pre-desiliconization-sodium carbonate activation-acid leaching process.The effects of sodium silicate solution modulus(n(SiO2)/n(Na2O)),calcination time and calcination temperature on the crystalline form of layered sodium silicate and the exchange capacity of calcium and magnesium were studied.According to GB/T4209-2008 "Industrial Sodium Silicate",the modulus of sodium silicate solution is determined;phase and chemical composition of fly ash were analyzed by Xray diffraction(XRD)and X-ray fluorescence spectroscopy(XRF);the structure of the layered sodium silicate was analyzed by nuclear magneticresonance spectroscopy(NMR),and the peak of the 29 Si NMR image was fitted by dmfit2015 to identify the purity of the crystal;the skeletal vibration characteristics and micromorphology of layered sodium silicate were analyzed by Raman spectroscopy and scanning electron microscopy(SEM).The results show that the modulus of sodium silicate solution and calcination temperature are significant factors affecting the crystalline form of layered sodium silicate.When n(SiO2)/n(Na2O)is 2.0,layered sodium silicate with ? content up to 92% calcined at 720 °C for 120 min.The exchange capacity of Ca2+ and Mg2+ is 358 mg/g and 430 mg/g,better than the value specified in GB/T19421-2008 "Test method for layered crystalline sodium disilicate".The ? phase in the Na2Si2O5 crystal phase is favorable for the exchange of Ca2+ and Mg2+,and the presence of the amorphous phase is not conducive to the exchange of Ca2+ and Mg2+.(2)Molecular sieve SAB-15 was synthesized by using sodium silicate solution prepared by CFB gray acid method,and the SBA-15 was modified with 3-aminopropyltriethoxysilane(APTES).Then,it was functionalized by using phosphoryl acetic acid(PAA)and diethylenetriamine pentaacetic acid dianhydride(DTPADA)to investigate the its adsorption properties of on rare earth elements(Ho,Yb).The structure of the material was characterized by Xray diffractometry(XRD),physical adsorption(BET)and fourier transform attenuated total reflection infrared spectrometer(ATR-FTIR).The change of rare earth elements content in the solution before and after adsorption was determined by inductively coupled plasma optical emission spectrometry(ICP-OES).The results show that the highly ordered SBA-15 can be synthesized.The pore structure of the material is not destroyed after surface functionalization.Under neutral conditions(pH=6.0),the adsorption efficiencies of PAA-SBA-15 on Ho and Yb were 86.3% and 94.4%,respectively.Under acidic conditions(pH=2.0),the adsorption efficiencies of DTPADA-SBA-15 on Ho and Yb were 93.1% and 89.0%,respectively.The competitive ions Al3+ and Ca2+,the adsorption efficiencies of DTPADA-SBA-15 adsorption Ho did not decrease when competitive ions Al3+ and Ca2+ wereintroduced in the adsorption process,and the functionalized SBA-15 showed good selectivity for Ho adsorption.The adsorption efficiencies of PAA-SBA-15 and DTPADA-SBA-15 remained at a high level after three cycles,indicating that the adsorbent material has good reusability.The adsorption kinetics of Ho and Yb by PAA-SBA-15 and DTPADA-SBA-15 accorded with the quasi-secondary kinetic model,and the adsorption behavior was chemisorption.The intraparticle diffusion model shows that the whole adsorption process is divided into two processes: boundary diffusion and intraparticle diffusion.