The thermal decomposition of S ¸aphane alunite from Ku ¨tahya, Turkey, has been studied using ambient-and high-temperature X-ray diffraction analysis, differential thermal analysis, and thermogravimetric analysis. The alunite content in the ore was about 47.5%. The first endothermic peak at about 545 °C is due to the dehydration of the structural water and the transformation of alunite into KAl(SO 4 ) 2 and amorphous Al 2 O 3 . The small exothermic peak at about 712 °C is probably caused by the recrystallization of KAl(SO 4 ) 2 into K 3 Al(SO 4 ) 3 and Al 2 -(SO 4 ) 3 . So far, no work has reported this new phase, K 3 Al(SO 4 ) 3 , resulting from the exothermic reaction of alunite. Moreover, the d and I values of unidentified peaks in the previous literature gave reflections that were surprisingly identical to those of the K 3 Al(SO 4 ) 3 peaks. The second endothermic peak at about 810 °C is due to the desulfurization by which K 3 Al(SO 4 ) 3 and amorphous Al 2 (SO 4 ) 3 decompose into K 2 SO 4 (arcanite) and two polymorphic varieties of Al 2 O 3 (κ and γ). Above 800 °C, only γ-alumina and arcanite were observed.
The dehydration kinetics of Şebinkarahisar (Gedehor) alunite ore in a fluidized bed was investigated. It was observed that the dehydration reaction rate increased with increasing temperature and decreasing water vapor pressure. However, the particle size and air flow rate did not have a significant influence on the dehydration rate. On the basis of these conversion−time relations, the dehydration process was assumed to be controlled by chemical reaction. By nonlinear regression analysis, the Avrami model was found to fit the kinetic data best. n ∼ 1, the high activation energy value (209 kJ mol-1), and scanning electron microscopic photographs of the calcined ore supported this control mechanism.
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