Preparation of industrial ammonium diuranate from a boltwoodite ore as a mediator for nuclear fuel and catalytic utilizations

Abstract

To date, uranium is one of the most crucial critical metals and a high supply-demand gap is forecasted in the coming years. However, the upsurging interest in uranium in nuclear fuel cycles prompted an engineered hydrometallurgical route capable of producing a high-grade ammonium diuranate [(NH4)2U2O7] capable of serving as an intermediate in nuclear base fuel and catalytic industries. Consequently, the processing of an indigenous boltwoodite ore containing admixtures of albite (Na2.00Al2.00Si6.00O16.00), boltwoodite (Na2.00K2.77U3.00Si6.00O9.00H4.00), thorite (Th4.00Si4.00O16.00), and quartz (Si6.00O6.00) by the hydrometallurgical process was examined in hydrochloric acid media. The experimental parameters during leaching including lixiviant concentration, reaction temperature, and particle size on the ore dissolution were investigated. At established conditions (2.5 mol/L HCl, 75 °C, 75 μm), 99.1% of the initial 10 g/L ore was dissolved within 120 min. The estimated activation energy of 17.30 kJ/mol indicated a mixed control reaction mechanism as the rate-determining step for the ore dissolution. The thermodynamic and kinetic tests were also examined to explore dissolution feasibility. The apparent change in enthalpy (ΔH = 73.95 kJ/mol) and negative values of Gibb's free energy change affirmed that the U(VI) extraction is endothermic and spontaneous for temperatures between 300 K and 348 K. Almost 99.3% U(VI) in the pregnant solution was quantitatively extracted using TBP in kerosene, and 97% of the total U(VI) was quantitatively precipitated using NH4OH solution. The precipitate was further crystallized to obtain industrial-grade ammonium diuranate comparable with the international standard specification of nuclear purity ASTM C 788–03; recommended to serve as a mediator in the nuclear fuel and catalytic industries.