Fundamental Applied Research On The Separation Of Alumina And Silica From High Alumina Fly Ash

High alumina fly ash(HAFA)is a characteristic secondary resource in China,with an annual output of more than 25 million tons,among which the alumina content can be as high as 50%.However,China is extremely short of bauxite resources,and the technology development of extraction alumina from HAFA is a major national demand.HAFA has a low mass ratio of alumina to silica(A/S?1.2),and silicon mainly exists in amorphous silica and crystalline mullite(3Al2O3·2SiO2).Alumina-silica separation is the key to the utilization of HAFA and has been extensively studied at home and abroad.But there are some problems such as the unclear decomposition mechanism of crystalline alumina-silica component in concentrated alkali system(sub-molten salt medium),the low removal rate of amorphous silica in HAFA,and the low utilization value of silicon components during the alumina-silica separation of HAFA by wet method.To this end,this thesis focused on the efficient separation of alumina and silica in HAFA.The research of the decomposition rule of crystalline alumina-silica component(mullite and corundum)in sub-molten medium,and the synergistic dissolution of HAFA and red mud in sub-molten salt medium were carried out.Meanwhile,a new method for deep separation of alumina and silica in HAFA by thermochemical method was also proposed,and relevant process optimization and application basic research were carried out systematically.The following results and progresses in this thesis were achieved:(1)The main aluminum-containing phases of HAFA are mullite and corundum,and the decomposition rule of mullite and corundum in NaOH sub-molten medium was investigated.The results showed that corundum is more difficult to decompose than mullite.Under the condition of Na2O concentration of 370 g/L and initial caustic ratio of 25,mullite began to decompose at 120 ?,while corundum began to decompose when the temperature was higher than 180 ?.The decomposition process of mullite and corundum were controlled by chemical reaction,and the corresponding apparent activation energies were 67.46 kJ/mol and 161.82 kJ/mol,respectively.At the same time,it was found that the rod-like structure of the dealuminized slag in the dissolution slurry of HAFA sub-molten salt method made it easy to separate the liquid and solid.Based on this,a new method of HAFA-red mud synergistic dissolution extraction of alumina was proposed,which effectively solved the filtration problem of red mud dissolution slurry of sub-molten salt method,and the slurry filtration speed was increased to 1.264 m3/(m2·h).Meanwhile,the synergistic dissolution of HAFA-red mud promoted the conversion of Na8(Al6Si6O24)(OH)2.04(H2O)2.66 to NaCaHSiO4,and inhibited the formation of 1.2Na2O·0.8CaO·Al2O3·2SiO2·H2O in dissolution slag.Finally,the alumina extraction rate in red mud was increased from 72.14%to 91.70%.(2)The reaction thermodynamics of the main phase of HAFA with ammonium fluoride were studied.It was found that the reaction of amorphous silica with ammonium fluoride could be carried out spontaneously when the temperature is higher than 74 ?.By studying the changes of the morphology,phase,valence bond and chemical composition of HAFA particles during the reaction of HAFA with ammonium fluoride at low temperature(120?160 ?),it was found that ammonium fluoride could react with amorphous silica without reacting with mullite and corundum,and a new separation technology of amorphous silica in HAFA by low-temperature ammonium fluoride thermochemical method was established.The effects of different factors(ratio of raw materials,reaction temperature and reaction time)on the desilication rate were investigated.Under the optimized conditions,the removal rate of silica could reach 59.38%,and the A/S of desilicated HAFA was inceased to 3.15.Mechanistic studies showed that the thermochemical reaction of HAFA and ammonium fluoride destroyed the Q4(0Al)structure and the lower activity Si-O-Al coordination structure in glass phase,which made the amorphous silica highly converted into water-soluble ammonium fluorosilicate.When the reaction product was treated with water,the ammonium fluorosilicate was dissolved into water to achieve the efficient removal of amorphous silica.Meanwhile,the desiliconization liquid was subj ected to ammoniation treatment to obtain silica product and ammonium fluoride,and the ammonium fluoride could be recycled.(3)Based on the reaction characteristics of ammonium fluoride capable of decomposing mullite at temperatures above 165 ?,a new process for high temperature thermochemical reaction separation of crystalline aluminum silicon in HAFA was proposed.In the pyrolysis process,the aluminum component was decomposed and converted into corundum(?-Al2O3),and the silicon component was converted into the by-product of ammonium fluorosilicate.The effect of factors on the separation process were investigated and the optimized conditions were obtained.Under the optimized conditions,the silica content in solid phase corundum product was less than 0.2 wt%.Meanwhile,the reaction process of thermochemical was clarified by analyzing the results of phase and composition,and the nano silica with specific surface area of 441.8 m2/g and over 99%purity was successfully prepared from ammonium fluorosilicate.