Work

Understanding the Thermal Decomposition of Mg(NO 3 ) 2 ·6H 2 O and Its Role in Sustainable MgO and NO x Recovery

Year: 2025

Type: article

Abstract: The thermal decomposition of magnesium nitrate hexahydrate (Mg(NO 3 ) 2 ·6H 2 O) to produce MgO and NO x gases is an important stage in closing the loop for the direct nickel process (a low environmental impact process capable of producing high-purity nickel and cobalt using laterite ores). Nitric acid is used for leaching the laterite minerals, while MgO is used for the precipitation of metal ions; therefore, recycling the MgO and nitric acid from the barren liquor, which is rich in hydrated forms of magnesium nitrate, enables this process to be sustainable as well as effective recycling of the sludge. However, this process demands an advanced understanding of the thermal decomposition mechanism of Mg(NO 3 ) 2 ·6H 2 O into MgO and NO x gases and key parameters that can control the thermal decomposition process, product optimization, and the reactivity of the MgO produced. This present work studied the thermal decomposition of Mg(NO 3 ) 2 ·6H 2 O at different heating rates using thermogravimetric analysis (TGA), TGA-infrared (TGA-IR), and in situ high-temperature X-ray diffraction (XRD), while the MgO produced using different conditions was characterized using XRD, SEM, N 2 –physisorption, XPS, and citric acid reactivity testing. The TGA, TGA-IR, and high-temperature in situ XRD results obtained indicate that thermal decomposition of Mg(NO 3 ) 2 ·6H 2 O to MgO follows a multistage process with multiple intermediate products formed. At lower heating rates, the multiple stages of decomposition were clearly observed without overlapping, while at higher heating rates, these stages started overlapping (5 versus 20 °C/min). Thermal decomposition temperatures and heating rates clearly demonstrated that surface area, porosity, and particle size of the MgO were modified. The results from the reactivity tests (for reaction between MgO and citric acid) using MgO produced using different heating rates and final reaction temperature showed that MgO produced at lower heating rates and lower final reaction temperature were observed to be reactive (ranges tested were 5–20 °C/min and 500–800 °C), and this was most likely due to these samples having higher surface area and pore volume.

Source: Industrial & Engineering Chemistry Research

DOI: 10.1021/acs.iecr.5c02714