Fundamental Research On The Extraction Of Alumina From High-alumina Coal Fly Ash By Hydrothermal Process

In China,25 million tons of high-alumina coal fly ash (HACFA) is emitted every year and above 100 million tons is piled up over the years. The HACFA causes seroius air, water and earth pollution. Meanwhile, due to the lack of high-grade bauxite, the degree of dependence on foreign bauxite is very high. Therefore, efficient extraction of alumina from HACFA not only resolves the the pollution caused by HACFA, but also decreases degree of dependence on foreign bauxite. The alumina-silica ratio of HACFA is low. Thus, tranditional Bayer Process is not fit for the treatment of HACFA. The hydrothermal Process could extract alumina from alumina-containing minerals in NaOH solution with CaO. The reaction parameters of hydrothermal process is mild, but the caustic ratio of alumia-extraction solution was usually high, and the alumina in solution was difficult to recover.The extraction of alumina from HACFA is considered as research object. Mechanism of alumina extraction by one-step hydrothermal process were studied, and optimized conditions were obtained. On the base of one-step process, two-step hydrothermal method was proposed, to decrease the caustic ratio of alumina-extraction solution. Then, the kinetics of alumina extraction and Na2O removal from alumina-extraction slag was studied. In this thesis, the following results and progresses were achieved.(1) The regulation of decomposition and transformation of HACFA in hydrothermal system was studied systematically. The effect of conditions, such as temperature, concentration of NaOH and calcium-silicon ratio on the extraction rate of alumina and caustic ratio was studied systemly. The result shows the conversion process of HACFA was followed by mullite, hydrosodalite, zeolite and sodium calcium silicate (NaCaHSiO4) successively. The formation of zeolite accounts for the main leaching loss of alumina. Elevating reaction temperature and liquid-solid ratio lower the content of zeolite, thus increasing the extraction ratio of alumina. Under the condition of 260 ℃,40% NaOH, liquid-solid ratio 12, calcium-silica ratio 1.0 and 45 minutes,91.3% of alumina was extracted from HACFA.(2) To lower the high caustic ratio of alumina extraction solution in one-step hydrothermal process, two-step hydrothermal method was put forward, which is composed of one-step hydrothermal process and the process of lowering the caustic ratio. After treated by two-step alkaline hydrothermal process, the caustic ratio of solution was decreased from 11.5 to 7.2. Compared with one-step hydrothermal process, the decrease of caustic ratio improved the efficiency of NaOH solution.(3) On the basis of decomposition mechanism of HACFA, the kinetic analysis of extracting alumina from HACFA by hydrothermal process was studied. The alumina-extraction process was divided by decomposition of mullite and decomposition of hydrosodalite. Decomposition kinetics of mullite was fitted by shrinking core model, and the surface reaction is determined as the main rate-limiting step. The resulting activation energy was determined as 29.70 kJ/mol. Nucleation& crystallization of NaCaHSiO4 was determined as the rate-controlling step of decomposition process of hydrosodalite. The kenitics was fitted by Avrami-Erofeev equation and the activation energy was 148.90 kJ/mol.(4) Removal of Na2O in alumina-extraction slag was explored by low-alkaline hydrothermal process to recycle the soda and utilize alumina-extraction slag. After the treatment by low-alkaline hydrothermal process, the slag was converted to short-chained tobermorite, and the residual Na2O content of 2.2%. The XRD, FTIR and NMR analysis showed that Na2O occurred in alumina-extraction slag in three forms:as unreacted NaCaHSiO4, adsorption due to the substitute of aluminum and adsorption by silicon hydroxyl. The residue was further washed with warm water, and the content of Na2O wasdecreased to 0.6%.