Influences of additives on the fate of uranium during coal co-combustion process: The significant activation by CaCO3

Abstract High-efficiency extraction of uranium (U) is a key to determining the feasibility of radioactive hazards reducing and nuclear resources supplementing. So in this study, the influences of additives on U extraction during combustion were shown via Tessier extraction procedures and leaching tests. By co-firing U-rich coal with alkali metal-bearing, alkaline earth-bearing, and transition metal-bearing additives, the proportions of active U are decreasing, increasing, and ambiguous, respectively, among which CaCO3 displays the most remarkable improvement on U extraction. Thus, combining aqueous experiments with some characterizations (e.g., X-ray diffraction, scanning electron microscopy-energy dispersive spectroscopy), the fates of U during combustion with/without CaCO3 were comparatively studied as the roles of times (0–120 min), temperatures (450–1050 °C), and CaCO3 dosages (0–30%). Changes mainly include three points as co-firing time rising: 1) the oxidation of organics is suppressive in the CO2 atmosphere (owing to CaCO3 thermolysis), so U ions strongly associating with organics cannot turn into UOX at times ≤12 min; 2) the favorable captures of acidic phases, including UOx, by CaO promote the generation of uranate and its redistribution in alkaline eutectics; 3) although the former changes stabilize the existence of active U, iron oxides compete for the capture of UOX, contributing to its encapsulation in acid-insoluble glass. After co-firing with 20% CaCO3 and acid extraction, despite ~23.7% of U remaining in residue, it is promising to extract U by the method here, given a considerable improvement (~22.0%) on U extraction only under 5% CaCO3.