| Extracting Al2O3from coal fly ash has strategic significance for sustainable development of China aluminum industry, resource guarantee and environmental protection. The current main methods of extracting Al2O3from coal fly ash include alkali method, acid method, the alkali-acid combination, and ammonium sulfate method. Although the acid method demands good corrosion resistance of apparatus materials, it has potential advantage to form emission reduction and circular process because its shorter process flow and less residues. Concentrated sulfuric acid roasting can sufficiently utilize high concentration, strong acidity and corrosivity of sulfuric acid to treat coal fly ash. The existing study of roasting process parameters, acid consumption and roasting reaction mechanism are not sufficient, and the integrated process as a whole from raw ash to Al2O3do not have solid scientific foundation and key technique parameters. Therefore, the whole process under this study was designed containing concentrated sulfuric acid roasting ash, clinker hot water leaching, Fe removal of leachate by two-step oxidation, Al2(SO4)3crystallization from leachate after Fe removal and Al2O3preparation by calcinating Al2(SO4)3. The mechanism and technical process of every step was analyzed and interpreted, and the fundamental data and theoretical knowledge obtained could be used for the process design and optimization of sulfuric acid roasting process to extact Al2O3from coal fly ash.The concentrated sulfuric acid roasting which is the key step of whole process was studied. The effect of single factor and orthogonal experiments on Al2O3extraction rate indicate that Al2O3extraction rate increases with appropriately increased sulfuric acid concentration, acid/ash mass ratio and roasted temperature, while the effect of roasted time is relatively small. The Al2O3extraction rate can reach92-95%with lower residue yield0.62via recommended optimal roasting parameters as sulfuric acid concentration80%, acid/ash mass ratio1.5:1, roasting temperature270℃and roasting time60min. The obtained experimental data supply important basis for optimal parameters choosing in scaling up experiments, decreasing acid consumption and evaluating corrosion resistance of apparatus materials.TG/DSC and XRD methods have been used to investigated reaction process and mechanism of concentrated sulfuric acid roasting, and the process can be divided to3steps discovered first time:Al(HSO4)3formation and water evaporation at80-206℃, formation of Al(HSO4)3and A12(SO4)3H2O at206-241℃, and A1:(SO4)3formation from decompositions of Al2(SO4)3H2O and Al(HSO4)3at241-304℃. Further, the values of apparent activation energy for the three steps are52.61,74.11, and96.08kJ/mol respectively, an activation energy average of values obtained by Kissinger differential and Ozawa integral methods. Using the kinetics parameters estimated, such as activation energy, frequency factor, reaction order, three non iso-thermal kinetic equations which correspond to three thermo-chemical reaction steps in the roasting process are established. The results can provide theoretical guideline and fundamental data for technique design and process optimation of roasting process in sulfuric acid roasting method for extracting Al2O3from coal fly ash.The leaching process and kinetics of hydrosoluble Al2(SO4)3in clinker was investigated, and the optimal leaching parameters obtained were leaching time60min, leaching temperature85℃, liquid/solid mass ratio9:1, raw ash grinding time60min and stirring speed150-200rpm. The leaching kinetics of clinker based on shrinking core model indicate that the rate-controlling step of leaching process is chemical reaction step corresponding to the apparent activation energy40.923kJ/mol as well as the kinetics equation was established. The effects of ultrasound on leaching process show that almost50%leaching time and10℃leaching temperature could be reduced compared to no ultrasound operation at the same Al2O3extraction rate, which reveal the useful effect of ultrasonic field on process intensification and technology energy saving.Fe removal process of leachate from clinker was performed via air-H2O2two steps oxidation precipitation. The pH of Fe3+hydrolytic precipitation in leachate is2.8-3.0calculated by solubility product Ksp and water ion-product Kw, and CaCO3is chosen for neutralizer in Fe removal process because CaSO4generated from it can aggregate Fe(OH)3colloid as core of heterogeneous nucleation. The systematically experimental results indicate that the Fe removal rate can achieve99.25%obtained through recommended operating conditions as air oxidation time time7h, air oxidation temperature85℃, air flow rate50-55mL/min, H2O2oxidation time30min, H2O2oxidation temperature25℃, H2O2concentration14%and H2O2dosage5mL, when the magnetic stirring rate800rpm in the whole process. Meanwhile, the Fe concentration in leachate can be decreased to8.8mg/L from1.185g/L, and this method can largely enhance Fe removal efficiency and process economy.The relationship between Al2(SO4)3solubility and temperature was obtained by static method. The supersolubility and metastable region of Al2(SO4)3 determined from direct method indicate that increasing temperature can increase width of metastable region, and cooling rate rise and decreased stirring speed can narrow and widen metastable region respectively. The crystallization process of clinker leachate show that the crystal molecule combined cluttered content as SO42-, H2O and H+make the filtration difficult when the solution initial concentration is too high. The Al2(SO4)3phase can be better separated from the impurity in leachate when the stirring speed and cooling rate are enhanced. The experiment of adding seed crystal can obtain satisfied Al2(SO4)3by adding very small amount seed crystal. The grade of Al2O3product calcinated from Al2(SO4)3at850-870℃is determined as98.71%, which satisfy the technical standard for Al2O3product and its impurity for use in non-ferrous industry. |
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