Thermal behavior of Al(NO3)3·9H2O and its application in preparing Al2O3 and regenerating HNO3

The new process ‘coal gangue/fly ash – HNO 3 pressure leaching (NAPL) –low-temperature pyrolysis’ was proposed by our team, which aimed to achieve the transformation of coal-based solid wastes from harmless to Al resource. For cost control, the regeneration of HNO 3 at a low cost was crucial for the process. Following NAPL of coal gangue and fly ash, massive amounts of Al(NO 3 ) 3 ·9 H 2 O were collected, with a decomposition temperature as low as 523 K. The energy consumption for low-temperature pyrolysis could be supplied by the combustion of residual carbon (6.96 wt%) during the thermal activity of coal gangue, allowing HNO 3 to be regenerated and recycled without extra heat supply . However, little research has been done on the pyrolysis behavior and kinetics of Al(NO 3 ) 3 ·9 H 2 O, which was examined in this study. The pyrolysis characteristics of Al(NO 3 ) 3 ·9 H 2 O were assessed by TG-DSC analysis. Further experiments and XRD/FTIR/MS analysis revealed that phase evolution of Al(NO 3 ) 3 ·9 H 2 O followed the sequence of Al(NO 3 ) 3 ·9 H 2 O→ Al(NO 3 ) 3 ·nH 2 O→ Al(NO 3 ) 3 ·3Al(OH) 3 ·5/2 H 2 O→ AlOOH→ Al 2 O 3 . Subsequently, the thermodynamic (∆ G , ∆ H , ∆ S ) and kinetic ( E a , A ) parameters were calculated based on the thermogravimetric data by the iso-conversional methods. And the mechanism models and functions were identified via the Malek method. Finally, the economic evaluation of pyrolysis indicated that 5.04 × 10 –3 Nm 3 of natural gas was consumed during pyrolysis of one mole Al(NO 3 ) 3 ·9 H 2 O, equivalent to 44.33 $ per ton of Al 2 O 3 . As a basic research of Al(NO 3 ) 3 ·9 H 2 O pyrolysis, this paper enhances related theories of the NAPL process.