Investigation of the thermal behavior of Mg(NO3)2·6H2O and its application for the regeneration of HNO3 and MgO

Abstract

The nitric acid pressure leaching (the NAPL) process has caught people’s attention owing to the increasing demand for nickel and cobalt, and the gradual depletion of nickel sulfide. In this process, iron, aluminum, nickel, and cobalt are separated successively by MgO. Therefore, a large amount of Mg(NO3)2·6H2O is generated, which is an important carrier to achieving the cycling of HNO3 and MgO via pyrolysis. However, the thermal behavior data of Mg(NO3)2·6H2O are insufficient. Hence, related experiments were carried out, which indicated four stages in this process with the absorption energy of 212.85 kJ/mol. Moreover, the hypotheses 3(Mg(NO3)2·nH2O) → Mg3(OH)4(NO3)2 + 4HNO3 (n = 4/3, the coexistence of Mg(NO3)2·2H2O and Mg(NO3)2) determining the most possible route to form Mg3(OH)4(NO3)2 was confirmed. The four–step model (A → B → C → D → E) showed a strong correlation with TG curves and can be described appropriately with the Fn, Fn, An, and An models, respectively. The molecular dynamic simulation revealed that dehydration and decomposition process include melting, dehydration, formation of anhydrous magnesium nitrate, and pyrolysis. Finally, a novel technical route for HNO3 and MgO regeneration was proposed with the natural gas consuming about 8.52 × 10-3 Nm3 per mole of Mg(NO3)2·6H2O.