Enabling chloride salts for thermal energy storage: implications of salt purity†
Molten salts for use as heat transfer fluids in concentrated solar or nuclear power plants have experienced a resurgence over the past decade with a special focus on chloride-based salt mixtures, particularly for use in concentrating solar power and fast-spectrum nuclear reactors. Salt purification, specifically oxide removal, is required even for high purity commercial salts and can be achieved using many different methods. Carbochlorination, however, proves most effective according to thermodynamics and produces a gaseous byproduct easily removed from the salt. A variety of carbochlorinating reagents and reagent combinations were evaluated for thermodynamic favorability in the removal of common impurities in MgCl2-based feedstock or coverage gases used in industrial systems. Carbon tetrachloride exhibited superior purification thermodynamics above the melting point of common MgCl2-based salt compositions. Salt with composition of 68 : 32 mol% KCl : MgCl2 was purified on the kilogram scale by sparging with carbon tetrachloride, reducing dissolved oxide to trace levels (42 μmol MgO/kg salt). Interestingly, the lower purity salts exhibited magnesium and oxygen presence along grain boundaries in the corrosion layers while the purified salts did not, highlighting the need for decreased oxide content. The lessened corrosivity of the highly purified salt suggests a proper salt treatment may reduce dependence on specialized materials for use with molten salts.