Fundamental Research Of Extracting Alumina From High-Alumina Coal Fly Ash Using NH₄HSO₄/H₂SO₄ Mixed Solution

High-alumina coal fly ash(HAFA)is a typical industry solid waste and its emission and accumulation are very huge.The HAFA contains 40%-50%of alumina and is a potential resource for extracting alumina.The comprehensive utilization of HAFA is taken into account as China's strategy.The developing of alumina extracting technology from HAFA not only has an important social and environmental benefits,but also can reduce the external dependency of bauxite in China.In view of alumina extraction at a low cost from HAFA,this dissertation proposes a new process of alumina extraction from HAFA using NH4HSO4/H2SO4 mixed solution.The leaching technological conditions,phase transformation,leaching kinetics and medium circulation were systematically investigated.Simultaneously,the crystallization of intermediate ammonium aluminum sulfate dodecahydrate in acidic medium and its thermal decomposition to prepare alumina were studied in detail.Based on the fundamental research,a conceptual process of extracting alumina from high-alumina coal fly ash using NH4HSO4/H2SO4 mixed solution was developed.The main contents and results are as follows:(1)Based on the physical properties system and ELECNRTL modeling in Aspen plus,the thermodynamic properties of NH4HSO4/H2SO4 mixed solution were studied.The leaching process was then investigated systematically and the alumina extraction efficiency can reach 88.9%in optimal conditions,realizing the efficient extraction of alumina.The changes of HAFA in the leaching process were systematically analyzed via X-ray diffraction(XRD),scanning electron microscopy-energy dispersive spectroscopy(SEM-EDS),fourier transform infrared spectrometer(FTIR),etc.Results showed the HAFA particles gradually decreased and the specific surface markedly increased;the mullite and corundum,which were the main form taken by alumina in HAFA,gradually decomposed to release the alumina and the residue mainly contained amorphous silica phase.(2)The alumina leaching kinetics in NH4HSO4 solution,H2SO4 solution and NH4HSO4/H2SO4 mixed solution from HAFA were studied with the shrinking core models,respectively.Results showed that the leaching of alumina in above three systems were all controlled by diffusion through the product layer with an apparent activation energy of 78.04±3.38 kJ/mol,89.33±2.27kJ/mol and 91.10±8.98kJ/mol respectively.The alumina extraction efficiency was expressed successfully with c[HSO4-],c[H3O+],temperature and time by using an empirical kinetic relationship.(3)Solubilities of NH4Al(S04)2·12H20 in the NH4+-Al3+-H3O+-HSO4--SO42--H2O system were measured at different temperatures,concentrations of sulfuric acid and NH4+/Al3+ molar ratios using a dynamic method.The electrolyte NRTL model embedded in Aspen plus was applied to model the solubilities and the new electrolyte pair parameters were obtained via regressing the experimental results.The total relative average deviations of temperatures,(NH4)2SO4 molalities,H2SO4 molalities and Al2(SO4)3 molalities were 1.04%,0.32%,0.26%and 0.45%respectively,indicating that the calculation values have good coherence with experiment data.With the newly obtained model parameters,the fundamental solubilities of NH4Al(SO4)2·12H2O in the NH4+-Al3+-H3O+-HSO--SO42--H20 system can be calculated precisely for its crystallization process.Meanwhile,the crystallization process of NH4Al(SO4)2·12H2O was studied by way of experiment and Aspen plus simulation.(4)The thermal decomposition of ammonium aluminum sulfate dodecahydrate was studied systematically by TG-DSC and TG-MS technologies and the results indicated that its thermal decomposition contains three stages.The influence of different atmosphere on the thermal decomposition was also investigated and the results showed that O2 in atmosphere could restrain the thermal decomposition process.Meantime,the kinetic analysis in air was conducted and the activation energy of decomposition in the 2ed and 3rd decomposed stage were 246.3kJ/mol and 286.3kJ/mol calculated by Flynn-Wall-Ozawa(FWO)method.The most probable kinetic mechanism function are R3(contraction spherical function)and D3(Jander equation)with the reaction types of three-dimensional phase boundary reaction and three-dimensional diffusion,respectively.(5)The medium circulation experiment was conducted and the alumina extraction efficiency can maintain 86-90%.The accumulation of impurities in medium circulation experiment were investigated in detail and iron element was the main impurity(1.0-4.5g/L in circulation solution),which gradually accumulated with the increasing of circulation number.Based on the medium circulation experiment,the principle process of alumina extraction from HAFA using NH4HSO4/H2SO4 mixed solution was proposed.The Aspen plus software was applied to simulate the whole process with the ELECNRTL model,and the four key steps(leaching,crystallization,thermal decomposition and medium recovery)were discussed and optimized in detail.At last,the direct consumption costs(raw material,fuel and power)were calculated in theory on the basis of the simulated results.