Investigation of the Behaviour of Sodium Oxide and Iron Oxides in Solar Salt Melt

Abstract

Molten nitrates, commonly known as Solar salt are widely used as a heat transfer and storage medium in thermal energy storage system due to lower corrosion and high thermal stability. The impurities discovered in the molten salt arise from the pipes, valves and tanks associated with the thermal energy storage system and when molten salt directly interacts with the impurities in the presence of atmospheric air, it accelerates corrosion that results in the formation of different type of oxides mainly iron oxides. The impurities in iron oxide consist of hematite and magnetite. The study investigates the behaviour of iron oxides (hematite and magnetite) in the molten salt upon the addition of the sodium oxide. The oven experiment was carried out for 750 hours and operated at an elevated temperature of 560 °C. The chemical analysis of the salt samples collected from the oven experiments was performed using titration and MP-AES. Additionally, thermal analysis such as DSC tests were also carried out in order to support the results of oven experiment. Fact sage software was employed to identify the possible reactions occurred during the oven and DSC experiments and understand the correlation between the outputs generated in these experiments. The oven experiment revealed that the influence of sodium oxide accelerates the dissolution of iron which led to formation of sodium ferrite. In contrast, the DSC experiment highlighted the role of the external atmosphere and its influence in the conversion of the magnetite to hematite. These results offer new insights on the behaviour of impurities and the evolution of different phases in solar salt.