The ultimate purpose of this study was to investigate the use of a Li-Ca mixture for direct reduction of actinide oxides to actinide metals at temperatures below 1500°C. For such a process to be successful, the products of the reduction reaction, actinide metals, Li2O, and CaO must all be liquid at the reaction temperature so that the resulting actinide metal can coalesce and be recovered as a monolith. Since the established melting temperature of Li2O is in the range of 1427 to 1700°C and the melting temperature of CaO is 2654°C, the Li2O-CaO (lithium oxide-calcium oxide) pseudobinary system was investigated in an attempt to identify the presence of low-melting eutectic compositions.
The results of our investigation indicate that there is no evidence of ternary Li-Ca-O phases or solutions melting below 1200°C. In the 1200 to 1500°C range utilizing MgO crucibles, there is some evidence for the formation of a ternary phase; however, it was not possible to determine the phase composition. The results of experiments performed with ZrO2 crucibles in the same temperature range did not show the formation of the possible ternary phase seen in the earlier experiment involving MgO crucibles, so it was not possible to confirm the possibility that a ternary Li-Ca-O or Li-Mg-O phase was formed. It appears that the Li2O-CaO materials reacted, to some extent, with all of the container materials, alumina (Al2O3), magnesia (MgO), zirconia (ZrO2), and 95% Pt-5% Au; however, to clarify the situation additional experiments are required.
In addition to the primary purpose of this study, the results of this investigation led to the following conclusions. First, the melting temperature of Li2O may be as low as 1250°C, which is considerably lower than the previously published values in the range 1427 to 1700°C. Second, lithium oxide (Li2O) vaporizes congruently. Third, lithium carbonate and Li2O react with 95% Pt-5% Au and also react with pure Pt. Fourth, it is likely that some or all of the past high-temperature phase behavior and vaporization experiments involving Li2O(s) at temperatures above 1250°C have actually involved Li2O(l). If these past measurements were actually measurements performed on Li2O(l) instead of the solid, the thermochemical data for phases and species in the Li-O system will require reevaluation.