Leach rates of uranium and thorium from murataite ceramics

Abstract

Samples of murataite-based ceramics with chemical composition (wt.%): 5 Al₂O₃, 10 CaO, 55 TiO₂, 10 MnO, 5 Fe₂O₃, 5 ZrO₂, 10 AnO₂ (An=Th, U) were produced by melting in both glassy carbon crucibles in a resistive furnace and a cold crucible followed by crystallization during slow cooling. All the ceramics were composed of predominant murataite polytypeswith 5-, 8-, and 3-fold elementary fluorite unit cell, minor crichtonite and pyrophanite/ilmenite, and rare grains of perovskite and pyrochlore. More than 95% of total Th and U enter the murataite polytypes. The murataite grains have zoned structure where the 5C, 8C and 8C polytypes form their core, middle zone and rim respectively. Leach rates of Th and U were determined using a single-pass-flow-through (SPFT) procedure at pH=2 and pH=6.5 at a temperature of 90±2 °C. Average Th leach rates from the ceramics produced in the resistive furnace and cold crucible were found to be 1.00×10^-5 g/(m²×d) and 1.97×10^-5 g/(m²×d). Uranium was found to be higher leachable element than thorium: its average leach rate was 1.42×10^-3 g/(m²×d). Higher leachability of U as compared to Th may be due to either occurrence of U in minor crichtonite phase with lower chemical durability or hydrolytic processes resulting in formation of higher leachable U(VI).