Potential scalability of a cost-effective purification method for MgCl2-Containing salts for next-generation concentrating solar power technologies

Abstract

Next-generation concentrating solar power (CSP) technology requires a high-temperature heat-transfer fluid and thermal energy storage media. Molten MgCl2-KCl-NaCl is considered a potential candidate salt due to its thermophysical properties. However, MgCl2 presents various challenges because of its hygroscopic nature. To keep corrosion under control, this molten chloride needs to remain free of hydrates and other impurities. Here, we have developed an effective purification method for MgCl2-containing salts by 1) combining known laboratory-scale, batch-style thermal and chemical purification processes to enable scalable, continuous-style processes for commercial next-generation CSP operation; 2) improving overall efficiency and minimizing the major corrosive impurity MgOHCl by optimizing key engineering parameters such as heating temperature/time and amount of elemental Mg addition; and 3) investigating the addition of halite (NaCl) to carnallite (KMgCl3) to reduce the liquidus temperature. Laboratory-scale results suggest that 1) adding 6.5 wt% of halite and less than 0.1 wt% of elemental Mg to commercial carnallite and 2) following a heating schedule to at least 650 °C with ~3 h of holding time at that temperature can produce a ternary MgCl2-KCl-NaCl salt composition with a low liquidus temperature of about 400 °C. It also reduces the presence of the corrosive impurity, MgOHCl, from ~1 to 2 wt.% to ~0.1 wt%. Findings of these key engineering parameters should provide a pathway toward a scalable, continuous-style salt-purification process at a scale of metric tons per hour that can produce a corrosion-controlled chloride molten salt for CSP applications.