(418f) Evaluation of Concentrations of Residual Water, Hydroxides, and Oxides in Molten Anhydrous CaCl2

Molten calcium chloride can be used as an electrolyte for reduction and electrorefining of actinides. However, its hygroscopic nature may present a barrier to this application. Simply heating the salt to its molten state may not entirely remove the water. At elevated temperature, the water may hydrolyze CaCl₂ to form calcium hydroxide or oxide—both of which are expected to have substantial solubility in the molten salt. Such impurities are expected to interfere with the designed oxidation/reduction reactions in the electrorefiner. In this study, we examined a number of methods for quantifying these contaminants—including thermogravimetric analysis, cyclic voltammetry, and acid-base titration. The starting material was commercially prepared ultradry CaCl₂, which was melted to 850°C. Thermogravimetric analysis of this salt indicated a 1 wt% mass loss on heating to 770°C, indicating a water content of at least that level. Cyclic voltammetry of the as-received CaCl₂ indicated the presence of a reduction peak that could be attributed to calcium hydroxide. To verify this assumption, current peak response to known additions of calcium hydroxide was recorded. The resulting correlation can be used to make in-situ measurements of calcium hydroxide concentration in molten CaCl₂. Acid-base titration yields the total concentration of Ca(OH)₂ and CaO in solidified samples of the salt. The combination of these three methods can be used to determine removable residual water as well as hydroxides and oxides that form from non-removable water. This presents a route for future work in CaCl₂ molten salts towards developing an effective drying process.