The Thermal Stability of Molten Lithium–Sodium–Potassium Carbonate and the Influence of Additives on the Melting Point

The thermal stability of a molten LiNaK carbonate salt, potentially suitable for thermal energy storage, was studied up to a temperature of 1000 °C. The salt investigated was the eutectic Li₂CO₃–Na₂CO₃–K₂CO₃ in the proportions 32.1–33.4–34.5 wt. % and the study was done by simultaneous differential scanning calorimetry (DSC)/thermogravimetric–mass spectrometric (TG–MS) analysis in gas atmospheres of argon, air, and CO₂. It was found that (i) under a blanket gas atmosphere of CO₂ the LiNaK carbonate salt is stable up to at least 1000 °C. (ii) In an inert atmosphere of argon, the salt evolves gaseous CO₂ soon after melting and begins to decompose at between 710 °C and 715 °C with acceleration in the CO₂ evolution rate from the melt. An increase in the rate of weight loss is also observed after 707 °C. (iii) Under a blanket atmosphere of air, the gaseous CO₂ evolution from the salt is observed to commence at 530 °C, the onset of decomposition detected by DSC analysis at 601 °C and the rapid rate of weight loss determined by TG analysis at 673 °C. The melting point of the LiNaK carbonate studied was between 400 °C and 405 °C. Thermodynamic modeling with Multi-Phase-Equilibrium (MPE) software developed in CSIRO Process Science and Engineering indicated that additives such as NaNO₃, KCl, and NaOH lower the melting point of the LiNaK carbonate eutectic, and this was experimentally verified.