Thermal Behaviors Of Silicon Aluminum Minerals And Desilication From Bauxite Ores By Thermochemical Activation(TCA Process)

Thermal behaviors of silicon aluminium minerals and desilication technology from bauxite ores by thermochemical activation (TCA) have been investigated by using TG, DTA, IR, XRD and 29Si, 27A1 MAS NMR in this thesis. Following conclusions are achieved:1. Thermal behaviors of silicon aluminium mineralsPyrophyllite: Under thermal treatments, dehydroxylation of pyrophyllite (Al2O3 4SiO2 H2O) takes place at the temperature of about 580℃. The six-coordinate of aluminium in octahedrons is converted into five-coordinate or four-coordinate on dehydroxylation of aluminium ions in oxygen octahedrons containing OH groups. However, the five-coordinate Al shifts into four-coordinate Al and five-coordinate Al again with the temperature increase. The bond angle of Si-O-Si (Al) in the silica tetrahedral sheet rises, and the linkage between atoms weakens. But the obtained dehydrated pyrophyllite has a layered structure similar to the initial pyrophyllite.The kinetic of dehydration reaction investigated by non-isothermal thermogravimetric analysis shows that the dehydroxylation of pyrophyllite agrees with the mechanism of randomnucleation: f(α) = (l-α)-[-ln(l-α)] 3. The value of the apparent activation energy E andapparent pre-exponential Arrhenius factor A of the reaction is attained as 128 362.3 Lmor'and 688 716.6 min'1, respectively. The kinetic equation of dehydration is:da 688716.6At the temperature of about 1100℃, the layered structure of dehydrated pyrophyllite collapses and is transformed into amorphous SiO2, but the reconstruction of amorphous substance above 1100℃ forms mullite (3Al203-2SiO2). As the temperature increases, the mulltization is enhanced, and the recrystallization of the amorphous SiO2 into cristobalite will occur when the temperature exceeds 1200℃.Kaolinite: During dehydroxylation at 420α~660α, the kaolinite (Al2O3-2SiO2-2H2O) is converted into a thermally stable product of meta-kaolinite. The six-coordinate of aluminium in oxygen octahedrons containing of OH groups change into five-coordinate or four-coordinate on dehydroxylation. Breaking of some bonds lead octahedrons in kaolinite to be destroyed and distorted, the increase of defect in crystal lattice results in the meta-kaolinite as X-ray amorphous substance.The kinetic results of dehydration reaction show that the mechanism of dehydroxylation of kaolinite changes when the thermal weight loss is about α=0.7 because the OH groups in interlamellar space associated with the alumina sheets lose more easily than those in the intralamellar space between the silica and alumina sheets. When 0