Research On Extraction Of Alumina And Other Useful Resources From High Aluminium Fly Ash

A new type of fly ash was recently found in the west of Inner Mongolia, of which the Al₂O₃ content (close to 50 wt%) is comparative to the middle content bauxite ores. According to the resource characteristics of this high aluminium fly ash, as well the market demand, the route was established for the extraction of Al₂O₃,SiO₂ and Ga₂O₃ step by step, at the same time, the residue was used for structural material. Geological methods was introduced to study the characteristics and cause of formation of the high aluminium fly ash,as well as the regularity of substance evolvement and its inner mechanism during the extraction of Al₂O₃, which aimed to lay a foundation for the establishment of the route to extract Al₂O₃ from the particular fly ash.In this thesis, series of analysis and test measures were applied, including optical and electron microscope, X-ray diffraction, chemical analyse, ICP-MS analysis, ²⁹Si MAS NMR spectra and TGA-DTA analysis, and Multy-knowledge such as geochemistry, mineralogy, coal science, coal combustion science, phychemistry, metallurgy and inorganic chemistry were introduced. Starting with the research of the substance source and cause of formation of the high aluminium . fly ash, conduct the inorganic geochemistry character investigation on the combustion coal of this fly ash- soft coal in late Paleozoic era from Junger coalfield in Inner Mongolia, then we find that this kind of fly ash mainly originates from the thin boehmite and kaolinite that disperses in the coal, and the formation of such minerals was resulted from the particular geological and geochemical condicition; The research on the phases and chemical composition, microstructure and distribution of some major and trace elements in the high aluminium fly ash indicates that this peculiar fly ash is a kind of valuable regenerative mineral resources, apart from alumina, which is also enriched in mullite crystallite, amorphous SiO₂ and useful substance-Ga. According to such characteristics of the fly ash, the brief technical route of cooperatively extracting Al₂O₃ and other useful substance was determined as follows: Firstly, the fly ash is treated by high concentration of NaOH solution under low temperature to extract amorphous SiO₂ and increase its Al/Si ratio, and the desiliconed solution obtained during this treatment can be used for the preparation of white carbon black. Then the alkali-lime-calcined method is used to extract Al₂O₃ from desiliconed fly ash.The main phychemical conditions in the technical route were optimized by series of orthogonal experiments and necessary complementary experiments, and it shows as follwings: (1) The optimized conditions of desiliconlization: concentration of NaOH , 25%; mass ratio of ash to alkali, 1:0.5; reaction temperature 95°C, and reaction time 4 hours. After reaction under the optimum conditions, the extraction efficiency of SiO₂ amounts to 41.8%, the Al₂O₃ content of the fly ash increases from 48.5% to 57.38% and Al₂O₃/SiO₂ mass ratio increases from 1.28 to 2.39. (2) The optimized conditions of treating the desiliconed fly ash: The optimized condtions for calcining the desiliconed fly ash: Ca ratio, 2:1; Na ration,0.95:1; temperature of calcination, 1150°C, heat preservation time, 60 minutes. And the optimized condtions for the extraction of NaAlO₂: temperature, 70°C; time for dissolution of NaAlO₂, 10 minutes. According to the conditions mentioned above, white carbon black and alumina content with nation quality criterion was obtained in laboratory, and that the extraction rate of Al₂O₃ was more than 90%; On the basis of experiments, we conduct the dynamic investigation on the reaction between NaOH solution and the glass of fly ash, and the thermodynamic investigation on the main calcined reactions of the desiliconlized fly ash-Na₂CO₃-CaCO₃; Finally, taking the main substance evolvement as the clue, we discussed the movement behaviour of Ga and some deleterious elements such as U,Th,Pb and Cr, which would provid the scientific guidance for the cooperative extraction of Al₂O₃,SiO₂ and Ga₂O₃, as well for the elimination of the potential harm to human or the environment resulted from these deleterious elements.