The regeneration of HNO3 and MgO via multistep pyrolysis of Mg(NO3)2•6H2O and thermodynamic analysis of critical intermediate product

The extraction of Ni from limonitic laterite by the NAPL (nitric acid pressure leaching) process has received significant attention. However, how to achieve continuous and suitable acid-base regeneration restricts the industrialization of the NAPL process. In this paper, spray pyrolysis technology promises to solve this problem. Hence, the phase evolution of the sample during the spray pyrolysis was studied, which is Mg(NO3)(2)center dot 6H(2)O -> Mg(NO3)(2)center dot 2H(2)O -> Mg-3(OH)(4)(NO3)(2) -> MgO. Raman spectra of Mg-3(OH)(4)(NO3)(2) were recorded for the first time and further prove the conclusion of the phase evolution. In addition, the thermal analysis of the critical intermediate product (Mg-3(OH)(4)(NO3)(2)) was performed, which indicates that the endothermic in the pyrolysis process is 194.35 kJ/mol, and is divided into two stages. The two stages both include the process of dehydration and denitration and the amount of dehydration and denitration in the second stage is relatively small. The reaction rate of stage two is slow and the temperature requirement is higher. Thermodynamic calculation results show that the R3 and An models could describe the whole process well. The related kinetic equations were obtained and the thermal stability of Mg-3(OH)(4)(NO3)(2) was predicted. Based on the above research, a two-step pyrolysis process is proposed by combining spray pyrolysis (similar to 823 K) and calcining pyrolysis (>= 693 K, 0.5 similar to 1.5 h). This route is simple, efficient, and continuous, which is conducive to industrialization and can successfully reduce at least 58.31% of NOx entering the compressor, thereby making the compressor smaller and less electricity consumption.