Preparation Of Alumina From High-alumina Fly Ash By Improved Acid And Alkali Combination Method: An Experimental Study

Fly ash is an industrial waste residue from power plant, and its discharge increases gradually as power industries' developement in China. The waste residue not only has a large discharge, but also is harm to environment and landscape. The Al₂O₃ content of high-alumina fly ash is more than 30%. It can be taken as the alumina-making raw material, which is a better method to reuse the waste residue high effectively, reduce utilizing bauxite and relax the Al₂O₃ requirement inland. In the thesis, an improved acid and alkali combination method, which was the combination of acid and alkali combination method and seed precipitation method, was developed. It was used to solve the much more alkali consumption problem to make circular use of sodium aluminate liquor in seed precipitation method. During the experiments, the high-alumina fly ash from a thermal-electric plant of the North China was used as raw material, with Al₂O₃ content up to 38.23% and the major phases were mullite and alminosilicate glass. With sodium carbonate as an auxiliary agent, the ratio of fly ash sodium carbonate (by weight) was 1∶0.85, then the mixtures were sintered at 880℃for 90min, and finally, the nepheline phase (including nepheline 93%) was produced, which could be dissolved in acid liquid. The dissociation proportion of the sintered mixtures of the fly ash was up to 97%.For separating alumina from silica, sulfuric acid was chosen as medium for dissolving the sintered materials. Based on theoretical analysis and the experiments, the silica extraction efficiency was about 95% and the alumina extraction efficiency was about 93% in optimal conditions. For removing ferric ion from the aluminium sulphate, firstly, 2mol/L Na₂CO₃ solution was entered into alumina liquor to adjust the pH value of the solution to arround about 6 and the amorphous alumina-iron precipitation was formed; and then, 8mol/L NaOH was enlisted to solve the precipitation. After filtrating and washing, a sodium aluminate liquor with little amount of ferric ion was got finally. For desilication deeply, the saturated limewater was joined into the gained sodium aluminate liquor and tiny calcium was removed by sodium carbonate solution, and ultimately almost pure sodium aluminate liquor wasgained with a Al₂O₃/ SiO₂ ratio (by weight) arround 2500. By phase analysis and thermodynamic calculations, it was indicated that Al(OH)₃ crystal could agglomerate by itself in this sodium aluminate liquor. During experiment, Al(OH)₃ seed were introduced into the purified solution to agglomerate Al(OH)₃ particles by seed precipitation and its seed precipitation ratio was about 50%. Finally,α-Al₂O₃ product was prepared by calcining of the Al(OH)₃ precipitate at 1200℃for 2h. Meanwhile, the theoretical analysis and experimental results demonstrated that it couldn't get rid of iron deeply only by adjusting the pH value of the acidic aluminium sulphate solution ranging from 5 to 6 and then entering polyacrylamide flocculant.XRD, IR, SEM, granularity analysis and chemical analysis methods were used to characterize the alumina product. The experimental results indicated that the preparedα-Al₂O₃ powder is irregular in shape with granular size less than 3μm in diameter, and quality of the product satisfy the China National Chemical Standard GB8178– 87.Compared with the lime sintered method and Bayer method, this technique has advantages such as low energy consumption, less alkali consumption, high value of the product, and no discharges of the solid and liquid wastes, and the exhaust gases, meanwhile, without consumption of bauxite and limestone resources, and therefore, satisfying the requirements of development of the circulation economy in China. This sort of high-alumina fly ash is a potential alumina resource in the future.