| The annual generation of high alumina fly ash(HAFA)in China is approximately 25 million tons,most of them are basically disposed in landfills,causing huge waste of resources and serious environmental pollution.Therefore,it is necessary to develop technologies for resource utilization of HAFA.The extraction of alumina from HAFA by sub-molten salt method can realize the efficient extraction of alumina,which has broad prospects in application.This dissertation focused on the resource utilization of alumina-extracted residue(AER)from HAFA by sub-molten salt method,aiming at the resorce utilization of silicon components.The applied basic researches of the change of silicon components in coal fly ash,the decomposition and transformation of AER,and the preparation of calcium silicate hydrate new materials were carried out.The innovative achievements are as follows:(1)For the first time,the component changing rule of fly ash generated from the combustion of different coals under various pressure conditions were systematically investigated.Under the same combusting conditions,US PRB sub-bituminous generated submicrometer particles(0?500 nm)with the largest peak size and the highest number concentration,which were 44.5 nm and 6.2×107#/cm3,respectively;India Chandrapur lignite took second place,which were 30.0 nm and 4.12×107#/cm3,respectively;while China Shanxi lignite generated submicrometer particles with the smallest peak size and the lowest number concentration,which were 13.6 nm and 1.78×106#/cm3,respectively.With an increase in pressure,for all three coals,the peak particle size of submicrometer particles increased and the total number concentration decreased.Moreover,US PRB coal fly ash mainly contained quartz,anhydrite,lime,and hematite,with a higher Ca content.India Chandrapur and China Shanxi coal fly ash mainly contained quartz and mullite.India Chandrapur coal fly ash specimens had the highest Si content,while China Shanxi coal fly ash specimens contained the largest amount of Al.For all three coal types,by increasing pressure,the more volatile species(Na,Mg,Fe,and Ca)were enriched in the submicrometer particles,and less volatile species(Si and Al)were also partially transferred into submicrometer particles.(2)The effects of particle size and reactant coating on the decomposition and transformation of AER to calcium silicate hydrate in sodium hydroxide diluted solution and the reaction mechanism were studied.By decreasing the particle size,the decomposition and transformation rate of AER could be enhanced obviously and the Na2O content of the products could be decreased effectively.The Na2O content of the product was decreased from 6.30 wt%to 2.44 wt%when the particle size of AER was decreased from 50-74 ?m to below 50 ?m.The results revealed that the decomposition reaction kinetics of AER(NaCaHSiO4)corresponded with shrinking unreacted core models.According to the kinetic study,the decomposition reaction of AER with larger particle size was mainly under product layer diffusion control,and that of AER with smaller particle size was mainly under chemical reaction control.Therefore,by decreasing the particle size,the effect of reactant coating and product layer diffusion was weakened,thus the decomposition reaction rate of AER could be enhanced obviously and Na2O content of the products could be decreased effectively.(3)A novel method for the decomposition and transformation of AER in concentrated sodium carbonate solution was presented.The optimal conditions were obtained,and the decomposition and transformation rule were revealed.Under optimal conditions of reaction temperature=180 ?,Na2CO3 concentration=170 g/L,liquid-to-solid ratio= 10 mL/g,and reaction time=2 h,a low Na2O content of 1.02 wt%of the products was achieved,which is lower than that(>2 wt%)of products from the decomposition and transformation of AER in diluted NaOH solution.The liquid-to-solid ratio of the reaction solution was 10 mL/g,and the solution after reaction had a higher concentration of Na2O(100 g/L),when Na+ solution was recycled to the alumina extraction process(Na2O=500 g/L),the evaporated quantity of water for production of alumina per ton was decreased from 30 m3 to 15 m3,approximately,and the concentration ratio for the evoporation of the Na+ solution was decreased from 25 to 5,compared with the NaOH diluted solution process,the energy consumption was greatly reduced.(4)New methods for adjusting and controlling the properties of calcium silicate hydrate were developed.By increasing Ca/Si(molar ratio of Ca and Si)from 0.8 to 1.4,the intensities of the diffraction peaks of tobermorite were decreased,the Na2O content of the product was decreased from 0.80 wt%to 0.48 wt%,the degree of polymerization of the silicate chains of tobermorite was decreased,more fragmentary products were formed,the particles became loose,the porosity was increased,and the thermal conductivity of the product was decreased.By increasing Al/Si(molar ratio of Al and Si)from 0.05 to 0.20,the intensities of the diffraction peaks of aluminum tobermorite were increased,the degree of polymerization of the silicate chains of tobermorite was increased because of the incorporation of aluminum into the silicate structure of tobermorite,the greater charge deficit due to the replacement of Si4+by Al3+ ions was compensated by increased adsorption or binding of Na+,the Na2O content of the product was increased from 0.73 wt%to 1.82 wt%,more fiber products were formed,the porosity was increased,and the thermal conductivity of the product was decreased.(5)The long nanofiber intertwined foshagite type calcium silicate hydrate was prepared from AER.The thermal insulation was molded,the density was as low as 157.16 kg/m3,the porosity was as high as 93.56%,and the thermal conductivity was as low as 0.0473 W/(m·K),which was superior to the thermal conductivity requirement(?0.058 W/(m·K))of 170#thermal insulation(Density ? 170 kg/m3)according to GB/T 10699-2015.In addition,the tobermorite type calcium silicate hydrate thermal insulation product was prepared from AER.The thermal conductivity of the product was 0.0586 W/(m-K),which was superior to the thermal conductivity requirement(?0.065 W/(m·K))of corresponding thermal insulation according to GB/T 10699-2015.(6)A novel non-autoclaved fiber reinforced calcium silicate board product was prepared from fibrous aluminum tobermorite type calcium silicate hydrate.The optimal conditions for preparation of fiber reinforced calcium silicate board were:moulding pressure of 45 MPa,pulp fiber content of 5 wt%,and cement content of 20 wt%.Under optimal conditions,the fiber reinforced calcium silicate board had a high flexural strength of 10.55 MPa and low thermal conductivity of 0.2424 W/(m-K),which were superior to the corresponding properties requirements according to national standard JC/T 564.1-2008.The toughing mechanisms of the fiber reinforced calcium silicate board were that the fibers and the matrix had good adhesion and penetration,the cement particles were uniformly filled in the pores of the calcium silicae board,and the solidification of cement increased cohesiveness between the fibers and the matrix,enhancing the ability of the board to withstand the shear failure and bending force. |
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