FORMATION AND SOME CHEMICAL PROPERTIES OF ALKALINE-EARTH METAL MONOURANATES

The formation of monouranates of calcium, strontium and barium by the reactions of their carbonates with U3O8 in various atmospheres of air, carbon dioxide and hydrogen and in vacuum was studied by means of thermogravimetry and X-ray diffraction. The reactivity of the carbonates was compared with each other using the initiation temperature of the reactions. The initiation temperatures of the reactions of CaCO3, SrCO3 and BaCO3 with U3O8 in air were 580, 470 and 410°C, respectively. The compositions of these products were CaUO3.969, SrUO3.997 and BaUO4.000. The initiation temperatures of the reactions in CO2 were slightly higher than those in air, but their order was the same as in air. From thermodynamic consideration, the reactions in air were deduced to proceed by the following successive reactions: MCO3 → MO + CO2; MO + UO 8 3 + 1 6O2 → MUO4, where M= Ca, Sr, Ba. When heated in air the rhombohedral compounds, CaUO4 and α-SrUO4, had a wide range of oxygen-nonstoichiometry, but the orthorhombic compounds, β-SrUO4 and BaUO4, were very nearly stoichiometric. The initiation temperatures in hydrogen reduction of the monouranates increased in the order: α-SrUO4 < CaUO4 < β-SrUO4 < BaUO4. The reduced uranates MUO4-x were easily oxidized by heating in air, but for CaUO4-x the stoichiometric composition was hardly obtained. © 1979.